Raise errors on new warnings from within xarray

[max-sixty]

Notes are inline.

• Closes Filter expected warnings in the test suite #8494
• User visible changes (including notable bug fixes) are documented in whats-new.rst

---

Done with some help from an LLM — quite good for doing tedious tasks that we otherwise wouldn't want to do — can paste in all the warnings output and get a decent start on rules for exclusions

[max-sixty]

Going to merge this as it's time-sensitive.

PRs that don't merge main back in may cause broken tests; I will endeavor to fix asap. And there are comments inline for how to deal with errors — post here or tag me on PRs if something comes up.

[keewis]

Was this supposed to close #8494? If so, that didn't work so you might have to close it manually.