Add Python 3.12.

[bdice]

Contributes to rapidsai/build-planning#40.

This PR adds Python 3.12 entries to conda/wheel build and test jobs.

To keep the total number of GPU test jobs fixed, I modified the Python versions used by some existing jobs.

See previous migration to Python 3.11: #176

[bdice]

I recommend we start rolling out Python 3.12 to RAPIDS repositories after these items:

• rapids-build-backend: Update RAPIDS Python packages to use rapids-build-backend build-planning#31
• C++ wheels, at least for most of RAPIDS: Support dynamic linking between RAPIDS wheels build-planning#33

[vyasr]

It's worth noting that while we have historically talked about "we can't claim to support anything we don't test", we already violate that rule by only testing a subset, even in nightlies. Therefore I'd argue that the fact that this PR brings us to an even smaller subset isn't categorically bad.