[Observability] 9.0.8 Release notes

[mdbirnstiehl]

Generate the 9.0.8 release notes for Observability, and publish them before or on the release day.

• Docs release issue: https://github.com/elastic/dev/issues/3313

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