Fleet and Agent 8.18.8 release notes

[karenzone]

Docs release manager: @alaudazzi
Tech Writer: @karenzone
Docs release issue: https://github.com/elastic/dev/issues/3314

Fleet and Agent release notes

• Release notes PR: Doc: Add Fleet and Agent release notes for 8.18.8 ingest-docs#1864
  • Add PR link to docs release issue

Content source (merge not required)

• Agent: Changelog for 8.18.8 elastic-agent#10266
• Fleet: NO CHANGES

No backports required