resolve dependency-file-generator warning, other rapids-build-backend followup

[jameslamb]

Description

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

#5804 was one of the earlier 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 cuml up with those changes:

• removes unused constants in ci/build* scripts
• uses --file-key instead of --file_key in rapids-dependency-file-generator calls
• uses --prepend-channel instead of --prepend-channels in rapids-dependency-file-generator calls
• ensures ci/update-version.sh preserves alpha specs

[jameslamb]

/merge