Make Changelog generation script git-based

[ockham]

Spun off from #28424:

We should maybe reconsider our source of truth for Changelog generation: Maybe we shouldn't query the list of issues/PRs for the milestone from the GitHub API. Instead, we have more reliable information right at our fingertips: git itself. We might just want to call git log with some arguments to get that exact kind of information (in a more reliable way than from GH's milestone, which just manually replicates that information). If we still want/need PR numbers etc, we can probably query those from the GH API, based on the Changelog obtained from git.