ensure update-version.sh preserves alpha specs

[jameslamb]

Description

Contributes to rapidsai/build-planning#31

Follow-up to #1502

• ensures that update-version.sh does not remove alpha specs like ,>=0.0.0a0 in pyproject.toml and conda environment files
• consolidates rapids-build-backend versions in dependencies.yaml
  • since I was pushing a new commit here anyway, figured I'd take the opportunity to include that simplification recommended in Use rapids-build-backend. cudf#15245 (comment)

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

Notes for Reviewers

Shouldn't this also be adding a test on the __git_commit__ and __version__ constants?

I'd said previously that I didn't include that in #1502, but looks like I actually did:

rmm/python/rmm/rmm/tests/test_version.py

Line 18 in 15c0466

def test_version_constants_are_populated():

[jameslamb]

/merge