cleanup(apm): remove old rum deprecations

[kruskall]

Description

deprecated rum config options have been removed in 8.0 so they no longer apply.
remove the deprecated options from docs

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#14126

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>

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[colleenmcginnis]

@kruskall how important is it to backport all the way to 8.0? In 8.12 we moved the APM guide content from apm-server / APM guide to observability-docs / Observability guide. We would have to open separate PRs for 8.0-8.11 if those need to be updated. (I can do this, but I wanted to check before I spent too much time on it!)

[kruskall]

I'd say 8.12+ should be enough IMO

cc @simitt for confirmation

[colleenmcginnis]

I'll take the other backports off this PR, and I can address them separately if needed.

[kruskall]

@colleenmcginnis sorry for the ping, should I go ahead and merge this ?

[colleenmcginnis]

@kruskall yes you can merge any time! I can keep an eye on the backports and fix any conflicts.

[kruskall]

thanks 🙇