Fleet and Agent 9.1.5 release notes

[karenzone]

Docs release manager: @nastasha-solomon
Tech Writer: @karenzone

Fleet and Agent release notes

Release note PRs:

• Elastic-agent PR: Changelog for 9.1.5 elastic-agent#10308

  • Forwardport to main: [main](backport #10308) Changelog for 9.1.5 elastic-agent#10362

• Fleet-server PR: Changelog for 9.1.5 fleet-server#5625

  • Forwardport to main: [main](backport #5625) Changelog for 9.1.5 fleet-server#5629

• Add PR links to docs release issue: https://github.com/elastic/dev/issues/3312

Forwardports 🎗️

• Verify that release notes PRs are forwardported to appropriate branches.
  Release notes are published from main, and are not live until the forwardport to main is complete. ❗

[karenzone]

Follow up work:

• [docs] Fix release notes elastic-agent#10481
• [docs] Fix release notes fleet-server#5670