[Observability] 9.1 release notes

[mdbirnstiehl]

Generate the 9.1 release notes for Observability, and publish them before or on the release day.

Feature freeze: June 24 2025
Merge release notes by: July 28 2025
Anticipated release date: July 29 2025
Docs release issue: TBD

To-do:

• Draft the release notes
• Add the release notes PR to the docs release issue
• Known issues:
  • Carry forward known issues that haven't been fixed yet
  • Update the summaries of known issues that were fixed in the current release (add fix details)
  • Document new known issues
• Merge the release notes PR