Improve min version policy workflow

[VeckoTheGecko]

Changes:

• Migrate the min_deps_check.py script, and associated workflow to instead use https://github.com/xarray-contrib/minimum-dependency-versions (especially after support pixi environments xarray-contrib/minimum-dependency-versions#42 is merged)
• Update the workflow to post (or update) an issue on failure
• Make it easy to locally test