ensure update-version.sh preserves alpha spec, add tests on version constants

[jameslamb]

Description

Contributes to rapidsai/build-planning#31

Follow-up to #2331

• 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)
• adds tests that __git_commit__ and __version__ constants are present and that __version__ is populated

[jameslamb]

/merge