[8.6](backport #3752) docs: remove false deprecation note for standalone apm-server

[mergify]

Closes elastic/apm-server#12796

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This is an automatic backport of pull request #3752 done by Mergify.

[mergify]

Cherry-pick of 38f04c9 has failed:

On branch mergify/bp/8.6/pr-3752
Your branch is up to date with 'origin/8.6'.

You are currently cherry-picking commit 38f04c90.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   docs/en/apm-server/upgrading-to-8.x.asciidoc

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally