Drop Python 3.10 support

[gforsyth]

Description

Contributes to rapidsai/build-planning#246

Finishes the work of dropping Python 3.10 support.

This project stopped building / testing against Python 3.10 as of rapidsai/shared-workflows#494
This PR updates configuration and docs to reflect that.

Followups before merging

Check that there are no remaining uses like this:

git grep -E '3\.10'
git grep '310'
git grep 'py310'

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[gforsyth]

/ok to test

[jameslamb]

/merge