[8.5] Manual prebuilt rule updates support notice (backport #4934)

[mergify]

Resolves #3622 by adding a note explaining how automatic updates for prebuilt rule are supported per Stack version.

Previews

• Upgrade Elastic Security — New subsection
• Install and manage Elastic prebuilt rules — Add new Note

Serverless docs

No serverless docs needed, since it's versionless and always on the latest version.

Backports

Backport this all the way back. At some point the Mergify backports will fail because we reorganized both pages; use the backport tool or manually open PRs for the rest.

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This is an automatic backport of pull request #4934 done by Mergify.

[mergify]

Cherry-pick of 9ad5d70 has failed:

On branch mergify/bp/8.5/pr-4934
Your branch is up to date with 'origin/8.5'.

You are currently cherry-picking commit 9ad5d705.
  (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/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/detections/prebuilt-rules-management.asciidoc
	both modified:   docs/upgrade/upgrade-security.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

[github-actions]

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