Ignore type-removal warnings in XPackRestTestHelper (#38431)

The backport of #38022 introduced types-deprecation warning for get/put template requests
that cause problems on tests master in mixed cluster scenarios. While these warnings are
caught and ignored in regular Rest tests, the get template requests in XPackRestTestHelper
were missed.

Closes #38412

Author: Christoph Büscher

x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/test/rest/XPackRestTestHelper.java

@@ -14,6 +14,7 @@
 import org.elasticsearch.common.xcontent.XContentHelper;
 import org.elasticsearch.common.xcontent.json.JsonXContent;
 import org.elasticsearch.test.ESTestCase;
+import org.elasticsearch.test.rest.ESRestTestCase;
 import org.elasticsearch.xpack.core.ml.MlMetaIndex;
 import org.elasticsearch.xpack.core.ml.job.persistence.AnomalyDetectorsIndex;
 import org.elasticsearch.xpack.core.ml.job.persistence.AnomalyDetectorsIndexFields;
@@ -76,7 +77,9 @@ public static void waitForTemplates(RestClient client, List<String> templateName
             ESTestCase.awaitBusy(() -> {
                 Map<?, ?> response;
                 try {
-                    String string = EntityUtils.toString(client.performRequest(new Request("GET", "/_template/" + template)).getEntity());
+                    final Request getRequest = new Request("GET", "_template/" + template);
+                    getRequest.setOptions(ESRestTestCase.allowTypeRemovalWarnings());
+                    String string = EntityUtils.toString(client.performRequest(getRequest).getEntity());
                     response = XContentHelper.convertToMap(JsonXContent.jsonXContent, string, false);
                 } catch (ResponseException e) {
                     if (e.getResponse().getStatusLine().getStatusCode() == 404) {