[8.16](backport #4844) Add known issue for APM Server version 8.6 and older with ES 8.15

[mergify]

Description

See elastic/apm-server#16084

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 elastic/apm-server#16084

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)

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This is an automatic backport of pull request #4844 done by [Mergify](https://mergify.com).

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[mergify]

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