[8.x](backport #4505) Raise prefer_ilm configuration as a known issue

[mergify]

Description

prefer_ilm setting was set to false in 8.15.1. This is not exactly a known issue but documenting it as one to raise awareness. Note that, prefer_ilm does not affect clusters using APM and upgrading to 8.15.x and the expectation was that the new guide would be followed for any new use-cases to customize ILM policies however, IaC based setups using the older way to customize ILM policies were affected due to the issue causing their newly created custom ILM configs to not apply.

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

NA

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 #4505 done by [Mergify](https://mergify.com).

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