[CI] MixedClusterClientYamlTestSuiteIT is failing on 6.x

[jbaiera]

Failing cases:
org.elasticsearch.backwards.MixedClusterClientYamlTestSuiteIT test {p0=msearch/10_basic/Least impact smoke test}

Reproduction:

./gradlew :qa:mixed-cluster:v6.4.3-SNAPSHOT#mixedClusterTestRunner \
  -Dtests.seed=5568D675297362E \
  -Dtests.class=org.elasticsearch.backwards.MixedClusterClientYamlTestSuiteIT \
  -Dtests.method="test {p0=msearch/10_basic/Least impact smoke test}" \
  -Dtests.security.manager=true \
  -Dtests.locale=sr-Latn-RS \
  -Dtests.timezone=America/Danmarkshavn \
  -Dcompiler.java=11 \
  -Druntime.java=8

error message:

Failure at [msearch/10_basic:68]: expected [2xx] status code but api [msearch] returned [400 Bad Request] [{"error":{"root_cause":[{"type":"illegal_argument_exception","reason":"request [/_msearch] contains unrecognized parameter: [max_concurrent_shard_requests] -> did you mean [max_concurrent_searches]?","stack_trace":"[request [/_msearch] contains unrecognized parameter: [max_concurrent_shard_requests] -> did you mean [max_concurrent_searches]?]; nested: IllegalArgumentException[request [/_msearch] contains unrecognized parameter: [max_concurrent_shard_requests] -> did you mean [max_concurrent_searches]?];

error logs: https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+6.x+multijob-unix-compatibility/os=opensuse/16/console

Probably related to 64ecb17

cc @estolfo

Adding search label since it's in an msearch test.

[elasticmachine]

Pinging @elastic/es-search-aggs

[javanna]

This was fixed. See a2e8b85 .