Add a CI job to ensure that `CHANGELOG` is updating in a PR

[John15321]

Add a Ci job that would check that the CHANGELOG file has gotten updated with a given Pull Request.
A simple git diff between the feature branch and the target branch will be enough.

Blocked by:

• Add a CHANGELOG file #32

[dawnofmidnight]

Personally, I think this is kind of a pain. There are tons of PRs that really don't need a changelog entry. Even if you relied on a maintainer adding some sort of a "skip news" label, that takes a while and causes CI to fail for little reason.

I think that it's enough to add it to the PR template and reviewers can check if the PR needs it.