[8.16] 8.16.1 Release notes (backport #6178)

[mergify]

Adds 8.16.1 release notes and makes the following changes:

• Removes the breaking change that was reported in 8.16 since it's more relevant to Kibana.
• Adds a resolution date and recommendation to upgrade to 8.16.1 for the following known issues that were reported in 8.16.0:
  • Alerts page crashes if you upgrade to 8.16 and access it in a non-default Kibana space
  • Attempting to edit an Elastic AI Assistant Knowledge Base index results in an error

Previews:

• 8.16.1 release notes
• 8.16.0 release notes

  ---

  This is an automatic backport of pull request 8.16.1 Release notes #6178 done by Mergify.

[github-actions]

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