Fleet and Agent 8.19.6 release notes

[karenzone]

Docs release manager: @KOTungseth
Tech Writer: @karenzone
Docs release issue: https://github.com/elastic/dev/issues/3345

Fleet and Agent release notes

• Release notes PR: Add Fleet and Agent release notes for 8.19.6 ingest-docs#1871
  • Add PR link to docs release issue

Content source (merge not required)

• Agent: Changelog for 8.19.6 elastic-agent#10715
• Fleet: Changelog for 8.19.6 fleet-server#5761

No forward or backports required