[main] (backport #12861) Add Elastic Agent 8.19.12 release notes

[mergify]

Adds Elastic Agent 8.19.12 release notes.

Checklist

• @ebeahan review this PR to make sure the correct items are included
• @elastic/ingest-docs review the language
• @elastic/ingest-docs open a PR in the ingest-docs repo combining the Elastic Agent and Fleet Server release notes targeting the 8.19 branch
• @ebeahan merge or close on release day. No forward or backports are needed.

• Note: Release notes for 8.x versions are published from the ingest-docs repo. Merging this PR won't hurt, but it is unnecessary.

• @elastic/ingest-docs merge PR in the ingest-docs repo. No forward or backports are needed.

Content will published as part of elastic/ingest-docs#1896

---

This is an automatic backport of pull request #12861 done by Mergify.

[mergify]

Cherry-pick of 36294d3 has failed:

On branch mergify/bp/main/pr-12861
Your branch is up to date with 'origin/main'.

You are currently cherry-picking commit 36294d3a4.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	new file:   changelog/8.19.12.asciidoc
	new file:   changelog/8.19.12.yaml
	deleted:    changelog/fragments/1769621019-synthetics-add-playwright-dependency-on-wolfi.yaml
	deleted:    changelog/fragments/1769802977-fix-otel-healthcheck.yaml

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by them: changelog/fragments/1770821590-update-otel-components-to-v0.145.0.yaml

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally