resolve dependency-file-generator warning, remove unnecessary rapids-build-backend configuration

[jameslamb]

Description

Contributes to rapidsai/build-planning#31
Contributes to rapidsai/dependency-file-generator#89

#15245 was one of the first rapids-build-backend PRs merged across RAPIDS. Since it was merged, we've made some small adjustments to the approach for rapids-build-backend. This catches cudf up with those changes:

• consolidates version-handling in ci/build_cpp.sh
• removes commit-file configuration in pyproject.toml
  • as of Automatically figure out where to write GIT_COMMIT rapids-build-backend#30, this is no longer necessary if the project's top-level directory is `{project_name}.replace("-", "_")
  • and anyway, it was changed from commit-file to commit-files in that PR, so commit-file was being silently ignored here
• uses --file-key instead of --file_key in rapids-dependency-file-generator calls

Checklist

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

[jameslamb]

/merge