Enable Past CI
# What does this PR do?

Enable Past CI
_The documentation is not available anymore as the PR was closed or merged._<|||||>> merge it ... and monitor failures. 

I read your comment too quickly. So far the Past CI will be triggered only on pushing to `run-past-ci*` branches. I ran it ~ June 20 however, and I opened #18181 today.

I think we can launch past CI monthly or even bimonthly. Please let me know if you have different opinion, @LysandreJik. Thanks.