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.

[github-actions]

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