Add a basic deprecation warning that the JSON format is changing in v9

[thecoop]

V9 has a different non-detailed errors format, modified by #90529

[elasticsearchmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[elasticsearchmachine]

Hi @thecoop, I've created a changelog YAML for you. Note that since this PR is labelled >deprecation, you need to update the changelog YAML to fill out the extended information sections.

[DaveCTurner]

LGTM except this is a deprecation so it needs a changelog entry - I adjusted the labels but you'll need to tweak the changelog it just added.

[thecoop]

@elasticmachine update branch

[thecoop]

The deprecation changelog will be added by #90529

[DaveCTurner]

The deprecation changelog needs to go into this PR so it gets backported to 8.x

[thecoop]

@elasticmachine update branch

[mosche]

The deprecation changelog needs to go into this PR so it gets backported to 8.x

The deprecation changelog is in the manual backport PR, so that should be fine 👍
Have you checked if the failed Serverless check is related, it looks possible. But that shouldn't block this PR.

https://gradle-enterprise.elastic.co/s/vrauj2k6yvf7m/tests/task/:qa:x-pack-rest-test:yamlRestTest/details/co.elastic.elasticsearch.qa.rest.ServerlessXpackRestIT/test%20%7Byaml%3Dprofiling%2F10_basic%2FTest%20flamegraph%20from%20profiling-events%7D?top-execution=1

[thecoop]

The serverless failure is an unrelated known issue