feat(lint)!: don't lint the donation key

[bepri]

After discussion with @mr-cal and @lengau, it was decided that it wasn't very helpful to enforce the donation key on a project. It is commonly ignored and entirely unwanted for proprietary snaps like Spotify, Discord, or Firefox.

This will break project builds that are currently ignoring this key.

---

• I've followed the contribution guidelines.
• I've signed the CLA.
• I've successfully run make lint && make test.
• I've added or updated any relevant documentation.
• I've updated the relevant release notes.

[mr-cal]

What do you think about silently ignoring donation in lint: ignore: metadata:?

I realize this may make the scope of work larger than a fly-by PR, but we don't have a deprecation warning in Snapcraft 8 that this key is going to be dropped. Ideally, someone who heeds deprecation warnings in Snapcraft 8 won't get surprised with a broken build in Snapcraft 9.