Infrastructure and maintenance

[dopplershift]

More things that should be cribbed from MetPy:

• [ ] Directly use the assign_milestone workflow from MetPy. Not sure if that will require changes there.
• [x] Re-use the nightly build workflow (?) Probably requires some more refactoring of that
• [x] Re-use the linting workflow?
• [x] MAYBE Re-use MetPy's actions for Conda, PyPI, Tests, and docs. Likely means moving those to their own repos, so it would be vastly preferable to just re-use the workflows. 😒
• [ ] Remove docs/requirements.txt--this is old and outdated, the current one is in ci/doc_requirements.txt
• [ ] Sync labels? We've improved these a bunch
• [ ] Issue forms
• [x] Update minimum versions
• [x] Rename master to main

~- [ ] Handling of minimum deps in setup.cfg and automatically including in workflow & docs~

Re-using MetPy's workflows may mean turning some of its composite actions into callable workflows instead. I think I misunderstood those to start.

[dcamron]

Bump with updates as I see them