Changelog for 8.19.4

[pierrehilbert]

What does this PR do?

Why is it important?

Checklist

• I have read and understood the pull request guidelines of this project.
• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool
• I have added an integration test or an E2E test

Disruptive User Impact

How to test this PR locally

Related issues

Questions to ask yourself

• How are we going to support this in production?
• How are we going to measure its adoption?
• How are we going to debug this?
• What are the metrics I should take care of?
• ...

[ebeahan]

Is adding this file intended?

[karenzone]

The content from this PR was ported to elastic/ingest-docs#1854, as outlined in the 8.x Fleet and Agent release notes process. This PR is no longer needed, and can be merged or closed.

Going forward, please let me know if you'd like for me to close or merge the changelog PR after I've harvested the content,