Fleet and Agent 8.18.7 release notes

[karenzone]

Docs release manager: @kosabogi
Tech Writer: @karenzone

Fleet and Agent release notes

• Release notes PR: Doc: Add Fleet Server and Elastic Agent release notes for 8.18.7 ingest-docs#1855
• Add PR link to docs release issue: https://github.com/elastic/dev/issues/3279

Content source

• Agent: Changelog 8.18.7 elastic-agent#9922
• Fleet: Changelog for 8.18.7 fleet-server#5507

No backports required

Post release

• Known issue: Doc: Add Known issue for 8.18.7: Agent stuck on failed upgrade ingest-docs#1856

No backports required