Consolidate mpl version checks

[rcomer]

Rationale

Closes #2237. I should have been more explicit with those imports when I added them.

I'm wondering if it's worth moving all these mpl version checks to a central place, like dask does with its numpy version checks. I guess it depends how often we need this sort of version gating.

Implications

[greglucas]

I think consolidating some of these would make sense. See also: #1748 for a desire to print the in-use versions out somewhere too

[rcomer]

OK I went for a clean sweep...

I put the checks in mpl.__init__ rather than mpl.geoaxes to avoid circular imports with geocollection and contour.

Edit: I also ran the tests against mpl main branch.