evaluate_data_frame REST test can fail from unexpected warnings

[jtibshirani]

Build scan: https://gradle-enterprise.elastic.co/s/vptinxha2wxme

Repro line:

./gradlew ':x-pack:plugin:yamlRestTest' --tests "org.elasticsearch.xpack.test.rest.XPackRestIT.test {p0=ml/evaluate_data_frame/Test classification given missing actual_field}" \
  -Dtests.seed=96A8B5076DF8D87B \
  -Dtests.locale=it \
  -Dtests.timezone=America/Guatemala \
  -Druntime.java=8

Reproduces locally?: no

Applicable branches: 7.13, likely others

Failure history: only one failure of its type so far

Failure excerpt:

java.lang.AssertionError: Failure at [ml/evaluate_data_frame:189]: got unexpected warning header [
    	299 Elasticsearch-7.13.0-SNAPSHOT-45067c011e99363d974ebf96244bec212663eada "this request accesses system indices: [.ml-config], but in a future major version, direct access to system indices will be prevented by default"
    ]
        at __randomizedtesting.SeedInfo.seed([96A8B5076DF8D87B:1EFC8ADDC304B583]:0)
        at org.elasticsearch.test.rest.yaml.ESClientYamlSuiteTestCase.executeSection(ESClientYamlSuiteTestCase.java:451)

[elasticmachine]

Pinging @elastic/ml-core (Team:ML)

[jtibshirani]

It looks like these deprecation warnings were added in #60945.