Security 9.1.5 release notes

[natasha-moore-elastic]

Generate the 9.1.5 release notes for Security and Endpoint, and publish them on the release day.

• Docs release issue: https://github.com/elastic/dev/issues/3312

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