[main] (backport #14451) Add Elastic Agent 8.19.16 release notes

[mergify]

Adds Elastic Agent 8.19.16 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
  • Doc: Add Fleet and Agent release notes for 8.19.16 ingest-docs#1911
• @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.

  ---

  This is an automatic backport of pull request Add Elastic Agent 8.19.16 release notes #14451 done by Mergify.

[mergify]

Cherry-pick of 0cff498 has failed:

On branch mergify/bp/main/pr-14451
Your branch is up to date with 'origin/main'.

You are currently cherry-picking commit 0cff49876.
  (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/8.19.16.asciidoc
	new file:   changelog/8.19.16.yaml

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by them: changelog/fragments/1776080404-update-otel-components-to-v0.149.0.yaml
	deleted by them: changelog/fragments/1776674464-my-test-fragment.yaml
	deleted by them: changelog/fragments/1777551882-drosiek-marker-stale.yaml
	deleted by them: changelog/fragments/1778016504-fix-config-manager-errors-poisoning-aggregate-state.yaml
	deleted by them: changelog/fragments/1778271360-Include-fleet-server-in-windows-arm64-packages.yaml
	deleted by them: changelog/fragments/1778595299-update-otel-components-to-v0.151.0.yaml
	deleted by them: changelog/fragments/1778719896-expose-latest_only-option-on-statewatch-rpc-to-skip-intermediate-states.yaml
	deleted by them: changelog/fragments/1778743723-akamaiseimreceiver-initial-release.yaml
	deleted by them: changelog/fragments/1779287880-edot-collector-fix-deprecated-warnings-on-names.yaml
	deleted by them: changelog/fragments/1779447372-Write-EDOT-internal-telemetry-to-a-dedicated-diagnostics-file.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