Test failing due to wrong content_type - 10_basic/Verify that we can still find things with the template

[pgomulka]

introduced by #65500

Build scan:
https://gradle-enterprise.elastic.co/s/cypeuyt4db6ow/tests/:qa:rolling-upgrade:v7.12.0%23oneThirdUpgradedTest/org.elasticsearch.upgrades.UpgradeClusterClientYamlTestSuiteIT/test%20%7Bp0=mixed_cluster%2F10_basic%2FVerify%20that%20we%20can%20still%20find%20things%20with%20the%20template%7D#1

Repro line:

./gradlew ':qa:rolling-upgrade:v7.12.0#oneThirdUpgradedTest' -Dtests.class="org.elasticsearch.upgrades.UpgradeClusterClientYamlTestSuiteIT" -Dtests.method="test {p0=mixed_cluster/10_basic/Verify that we can still find things with the template}" -Dtests.seed=CCF20BC12765F837 -Dtests.security.manager=true -Dtests.bwc=true -Dtests.locale=he-IL -Dtests.timezone=PRC -Druntime.java=11 -Dtests.rest.suite=mixed_cluster	

Reproduces locally?:
yes

Applicable branches:

Failure history:

Failure excerpt:

"type" : "general_script_exception", |  
-- | --
  | "reason" : "Failed to compile stored script [test_search_template] using lang [mustache]", |  
  | "stack_trace" : "org.elasticsearch.script.GeneralScriptException: Failed to compile stored script [test_search_template] using lang [mustache] |  
  | at org.elasticsearch.script.ScriptCache.compile(ScriptCache.java:121) |  
  | at org.elasticsearch.script.ScriptService.compile(ScriptService.java:339) |  
  | at org.elasticsearch.script.mustache.TransportSearchTemplateAction.convert(TransportSearchTemplateAction.java:100) |  
  | at org.elasticsearch.script.mustache.TransportSearchTemplateAction.doExecute(TransportSearchTemplateAction.java:69) |  
  | at org.elasticsearch.script.mustache.TransportSearchTemplateAction.doExecute(TransportSearchTemplateAction.java:48) |  
  | at org.elasticsearch.action.support.TransportAction$RequestFilterChain.proceed(TransportAction.java:88) |  
  | at org.elasticsearch.action.support.TransportAction.execute(TransportAction.java:64) |  
  | at org.elasticsearch.tasks.TaskManager.registerAndExecute(TaskManager.java:175) |  
  | at org.elasticsearch.client.node.NodeClient.executeLocally(NodeClient.java:108) |  
  | at org.elasticsearch.client.node.NodeClient.doExecute(NodeClient.java:88) |  
  | at org.elasticsearch.client.support.AbstractClient.execute(AbstractClient.java:386) |  
  | at org.elasticsearch.script.mustache.RestSearchTemplateAction.lambda$prepareRequest$1(RestSearchTemplateAction.java:77) |  
  | at org.elasticsearch.rest.BaseRestHandler.handleRequest(BaseRestHandler.java:101) |  
  | at org.elasticsearch.rest.RestController.dispatchRequest(RestController.java:268) |  
  | at org.elasticsearch.rest.RestController.tryAllHandlers(RestController.java:353) |  
  | at org.elasticsearch.rest.RestController.dispatchRequest(RestController.java:198) |  
  | at org.elasticsearch.http.AbstractHttpServerTransport.dispatchRequest(AbstractHttpServerTransport.java:336) |  
  | at org.elasticsearch.http.AbstractHttpServerTransport.handleIncomingRequest(AbstractHttpServerTransport.java:401) |  
  | at org.elasticsearch.http.AbstractHttpServerTransport.incomingRequest(AbstractHttpServerTransport.java:318) |  
  | at org.elasticsearch.http.netty4.Netty4HttpRequestHandler.channelRead0(Netty4HttpRequestHandler.java:42) |  
  | at org.elasticsearch.http.netty4.Netty4HttpRequestHandler.channelRead0(Netty4HttpRequestHandler.java:28) |  
  | at io.netty.channel.SimpleChannelInboundHandler.channelRead(SimpleChannelInboundHandler.java:99) |  
  | at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) |  
  | at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) |  
  | at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357) |  
  | at org.elasticsearch.http.netty4.Netty4HttpPipeliningHandler.channelRead(Netty4HttpPipeliningHandler.java:58) |  
  | at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) |  
  | at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) |  
  | at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357) |  
  | at io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:103) |  
  | at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) |  
  | at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) |  
  | at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357) |  
  | at io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:103) |  
  | at io.netty.handler.codec.MessageToMessageCodec.channelRead(MessageToMessageCodec.java:111) |  
  | at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) |  
  | at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) |  
  | at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357) |  
  | at io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:103) |  
  | at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) |  
  | at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) |  
  | at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357) |  
  | at io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:103) |  
  | at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) |  
  | at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) |  
  | at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357) |  
  | at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:324) |  
  | at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:296) |  
  | at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) |  
  | at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) |  
  | at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357) |  
  | at io.netty.handler.timeout.IdleStateHandler.channelRead(IdleStateHandler.java:286) |  
  | at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) |  
  | at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) |  
  | at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357) |  
  | at io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:103) |  
  | at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) |  
  | at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) |  
  | at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357) |  
  | at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1410) |  
  | at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) |  
  | at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) |  
  | at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:919) |  
  | at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:163) |  
  | at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:714) |  
  | at io.netty.channel.nio.NioEventLoop.processSelectedKeysPlain(NioEventLoop.java:615) |  
  | at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:578) |  
  | at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:493) |  
  | at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:989) |  
  | at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74) |  
  | at java.base/java.lang.Thread.run(Thread.java:834) |  
  | Caused by: java.lang.IllegalArgumentException: No encoder found for MIME type [application/json; charset=UTF-8] |  
  | at org.elasticsearch.script.mustache.CustomMustacheFactory.createEncoder(CustomMustacheFactory.java:92) |  
  | at org.elasticsearch.script.mustache.CustomMustacheFactory.<init>(CustomMustacheFactory.java:73) |  
  | at org.elasticsearch.script.mustache.MustacheScriptEngine.createMustacheFactory(MustacheScriptEngine.java:96) |  
  | at org.elasticsearch.script.mustache.MustacheScriptEngine.compile(MustacheScriptEngine.java:75) |  
  | at org.elasticsearch.script.ScriptCache.lambda$compile$0(ScriptCache.java:110) |  
  | at org.elasticsearch.common.cache.Cache.computeIfAbsent(Cache.java:435) |  
  | at org.elasticsearch.script.ScriptCache.compile(ScriptCache.java:99) |  
  | ... 70 more