Fleet and Agent 9.1.6 release notes

[karenzone]

Docs release manager: @KOTungseth
Tech Writer: @karenzone

Fleet and Agent release notes

Release note PRs:

• Elastic-agent PR: Changelog for 9.1.6 elastic-agent#10714
  (Add forwardport-main label to original PR)

  • Forwardport to main:

• Fleet-server PR:
  (Add forwardport-main label to original PR)
  No user-facing changes, but we still need a PR to that effect.

  • Forwardport to main:

• Add PR links to docs release issue: https://github.com/elastic/dev/issues/3346

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. ❗