Security 8.19 release notes

[natasha-moore-elastic]

Generate the 8.19 release notes, and publish them before or on the release day.

Feature freeze: Tue, Jun 24 2025
Merge release notes by: Mon, July 28 2025
Anticipated release date: Tue, 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 Security release notes PR