[8.15] Remove known issue for AI Assistant from the 8.15.1-8.16.1 release notes (backport #6190)

[mergify]

Fixes https://github.com/elastic/security-docs-internal/issues/54 and makes a couple of minor adjustments:

• Re-adds manual rule run known issues to 8.16.1
• Removes an extra image

Previews:

• 8.15.x release notes
• 8.16.x release notes

  ---

  This is an automatic backport of pull request Remove known issue for AI Assistant from the 8.15.1-8.16.1 release notes #6190 done by Mergify.

[mergify]

Cherry-pick of b1dec23 has failed:

On branch mergify/bp/8.15/pr-6190
Your branch is up to date with 'origin/8.15'.

You are currently cherry-picking commit b1dec23d.
  (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)

Changes to be committed:
	modified:   docs/release-notes/8.15.asciidoc

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/release-notes/8.16.asciidoc

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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