Doc: Add Fleet and Agent release notes for 8.19.9 and 8.19.10

[karenzone]

Content sourced from changelogs and combined into one PR for the 8.19 branch

8.19.10

Agent: elastic/elastic-agent#12178
Fleet: elastic/fleet-server#6169 (no changes)

8.19.9

Agent: elastic/elastic-agent#11782
Fleet: elastic/fleet-server#6061 (no changes)

No forward- or backports required for this PR

Source PRs can be closed or merged.

PREVIEW:: https://ingest-docs_bk_1880.docs-preview.app.elstc.co/diff

[github-actions]

🤖 GitHub comments

Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[github-actions]

A documentation preview will be available soon.

• 🔨 Buildkite builds
• 📚 HTML diff
• 📙 Preview page

Request a new doc build by commenting

• Rebuild this PR: run docs-build
• Rebuild this PR and all Elastic docs: run docs-build rebuild

_{run docs-build is much faster than run docs-build rebuild. A rebuild should only be needed in rare situations.}

_{If your PR continues to fail for an unknown reason, the doc build pipeline may be broken. Elastic employees can check the pipeline status here.}