ci: deprecate ci/config/allow-ref script

ci: deprecate ci/config/allow-ref script

Now that we have the CI_BRANCHES mechanism, there is no need for anybody
to use the ci/config/allow-ref mechanism. In the long run, we can
hopefully remove it and the whole "config" job, as it consumes CPU and
adds to the end-to-end latency of the whole workflow. But we don't want
to do that immediately, as people need time to migrate until the
CI_BRANCHES change has made it into the workflow file of every branch.

So let's issue a warning, which will appear in the "annotations" section
below the workflow result in GitHub's web interface. And let's remove
the sample allow-refs script, as we don't want to encourage anybody to
use it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>