ci/github-script/prepare: avoid running CI when targeting channel branches

This moves the no-channel-base check into the prepare script to exit
early and prevent all of CI to run against those branches. We also
provide better output by posting a "Changes Requested" review, using the
existing infrastructure from the old cherry-picks check.

The review will be dismissed automatically once the branch has been
corrected, because the commits check will run and do it.

0 files changed, 0 insertions, 0 deletions