"PUT mapping with _doc on an index that has types" fails

[dnhatn]

This test fails twice after it was backported into 6.x today.

We expect the test fails with "the final mapping would have more than 1 type" but the actual reason is "Document mapping type name can't start with '_', found: [_doc]".

FAILURE 0.29s | MixedClusterClientYamlTestSuiteIT.test {p0=indices.put_mapping/20_mix_typeless_typeful/PUT mapping with _doc on an index that has types} <<< FAILURES!
   > Throwable #1: java.lang.AssertionError: Failure at [indices.put_mapping/20_mix_typeless_typeful:73]: the error message was expected to match the provided regex but didn't
   > Expected: the final mapping would have more than 1 type
   >      but: was "{root_cause=[{type=invalid_type_name_exception, reason=Document mapping type name can't start with '_', found: [_doc], stack_trace=InvalidTypeNameException[Document mapping type name can't start with '_', found: [_doc]]\n\tat 

I can reproduce this after around 10 runs.

./gradlew :qa:mixed-cluster:v6.0.1#mixedClusterTestRunner   \
-Dtests.seed=A3A03AA8BC85CA20   \
-Dtests.class=org.elasticsearch.backwards.MixedClusterClientYamlTestSuiteIT   \
-Dtests.method="test {p0=indices.put_mapping/20_mix_typeless_typeful/PUT mapping with _doc on an index that has types}"   \
-Dtests.security.manager=true   \
-Dtests.locale=de-GR   \
-Dtests.timezone=Asia/Baghdad   \
-Dcompiler.java=11   \
-Druntime.java=8

Relates #37871

CI: https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+6.x+internalClusterTest/284/console

[elasticmachine]

Pinging @elastic/es-search

[jpountz]

Thanks @dnhatn I'll look.