Fleet and Agent 8.19.4 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 8.19.4 ingest-docs#1854
• Add PR link to docs release issue: https://github.com/elastic/dev/issues/3280

Content source (merge not required)

• Agent: Changelog for 8.19.4 elastic-agent#9921
• Fleet: Changelog for 8.19.4 fleet-server#5506

No backports required