fix(changelog): detect deleted release sections

[teddygood]

Connections
Fixes #9245.

Description
The changelog check previously only used post-change line positions, which let deletions of already-released sections slip through. This switches the check to track both old and new hunk positions and adds a regression test.

[ErichDonGubler]

I'll take on reviewing this, though I likely won't be able to finish a review until early next week.

I've updated the OP to remove the PR checklist, since it's not useful here.

[ErichDonGubler]

Hey there! I've let this PR languish, and I apologize. I've failed to honor reasonable expectations for review. wgpu maintainership expects better WRT communication, and that's on me.

It's been hard to prioritize, with (1) an international move of me and my whole family with young kids, (2) other more pressing issues in the repo, and (3) a recent development with #9779 noting that we might move away from a single CHANGELOG file to having multiple RustDoc modules specifically for documentation.

I think this code is good enough that I'm going to merge it. Thanks for your first contribution to wgpu! 🌟❤️