[8.x](backport #4784) [AWS Firehose] Clarify where to find ES endpoint

[mergify]

Description

When setting up an AWS Firehose data stream, users should use the Elasticsearch endpoint URL that contains the .es subdomain.

While the non-.es URLs currently work and will continue to function, the URL containing the .es subdomain is designed as the dedicated endpoint for connecting to Elasticsearch. These URLs are the best option for future-proofing your setup, and we should recommend using them.

Documentation sets edited in this PR

Check all that apply.

• Stateful (docs/en/observability/*)
• Serverless (docs/en/serverless/*)
• Integrations Developer Guide (docs/en/integrations/*)
• None of the above

Related issue

Closes #

Checklist

• Product/Engineering Review
• Writer Review

Follow-up tasks

Select one.

• This PR does not need to be ported to another doc set because:
  • The concepts in this PR only apply to one doc set (serverless or stateful)
  • The PR contains edits to both doc sets (serverless and stateful)
• This PR needs to be ported to another doc set:
  • Port to stateful docs: <link to PR or tracking issue>
  • Port to serverless docs: <link to PR or tracking issue>

---

This is an automatic backport of pull request #4784 done by [Mergify](https://mergify.com).

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