Update docs after migrating from apm integration package to ES apm-data plugin in 8.15

[carsonip]

Remove mentions of integration packages as apm-server moved away from it in 8.15

[github-actions]

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[mergify]

This pull request does not have a backport label. Could you fix it @carsonip? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[carsonip]

Superseded by #4239