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.
Logging objects to stdout is not possible with `core.info`, so we
fallback to `console.log` instead. There's no functional difference for
these anyway.
This just moves the code over to ci/github-script to make it easy to
test and iterate on locally.
The name `prepare` is chosen, because the script will be extended with
the other steps from "PR / prepare" next.
