PR template: Ask release notes for incompatible changes, not bug fixes

[edolstra]

Motivation

We never mention bug fixes in the release notes, but we should include incompatible changes.

Context

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or bug fix: updated release notes

[fricklerhandwerk]

I think this should have been discussed on the team.
It's true that we don't currently write release note about bug fixes, but there may be good reasons to start doing so.
Whatever we decide, we should at least provide a reason why we do it that way.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/tweag-nix-dev-update-44/25546/1