[8.17](backport #45073) Fix Elasticsearch output retry backoff when receiving 429s

[mergify]

See #36926. This fix has two components:

• Return an error from Publish when the Elasticsearch output gets a 429 (too many requests) from Elasticsearch. This triggers a retry delay and reconnection attempt in the pipeline.
• Break the backoff counters for Publish and Connect into separate values, so a successful Connect call (which for Elasticsearch just means that an empty http GET gave an ok response) doesn't reset the exponential backoff for bulk ingest requests when they are being throttled.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

This comment on the issue has local testing instructions.

Related issues

• Fixes Publish() should backoff if Elasticsearch returns 429 HTTP rate limiting responses #36926.

  ---

  This is an automatic backport of pull request Fix Elasticsearch output retry backoff when receiving 429s #45073 done by Mergify.

[mergify]

Cherry-pick of 8b25d5b has failed:

On branch mergify/bp/8.17/pr-45073
Your branch is up to date with 'origin/8.17'.

You are currently cherry-picking commit 8b25d5b14.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   CHANGELOG.next.asciidoc
	modified:   libbeat/outputs/backoff.go
	modified:   libbeat/outputs/elasticsearch/client_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   libbeat/outputs/elasticsearch/client.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[mergify]

This pull request has not been merged yet. Could you please review and merge it @faec? 🙏