.github/workflows: process PRs in ascending order

.github/workflows: process PRs in ascending order

The Ceph repo has many PRs and we cannot process all the PRs with the
default "operations-per-run" value (30). At the time of writting the bot
processes 408 every day and there are around 938 PRs.
Even the job informs us that not enough PRs might have been processed
and encouraged us to increase "operations-per-run" if possible. However
it might expose us to Github's API rate limit.
So let's operate with the oldest PRs first, this should close a bunch of
PRs already. If not enough we can try to increase "operations-per-run".

Signed-off-by: Sébastien Han <seb@redhat.com>