Update minimum supported version policy checker

[VeckoTheGecko]

Fixes #2429

We had a GitHub actions workflow (adopted from xarray) to check that our minimum supported versions were according to SPEC 0.

This PR simplifies that workflow:

• Uses https://github.com/xarray-contrib/minimum-dependency-versions (which xarray now also uses)
• Instead of using the github actions workflow provided by the link above, it uses UV (this makes is super easy to run locally - which in this case one would want to do often)

uv run --with 'minimum-dependency-versions @ git+https://github.com/xarray-contrib/[email protected]'\
          python -m minimum_versions validate\
          --policy=.github/ci/policy.yaml\
          --manifest-path=pixi.toml\
          pixi:test-minimum

Other changes:

• Update minimum version pins to align policy