Implement changelog check for merge queue

[Dav1dde]

The merge queue trigger in github actions does not have access to all included PRs, hence changelogs can't be verified the same way we do it within the scope of a PR.

But the action has access to two commit hashes (before + after), the action can grab a diff of the two commits and make sure there are only changes in the Unreleased section.

This will finally ensure no PR with the changelog in the wrong section can be merged.