ANN: milestone_checker now a required check in astropy

[Pey Lian Lim]

To whom it may concern,

I have just made a job called "milestone_checker" required to pass for merging a PR into main or the v6.0.x backport branch. This is to ensure that our PRs have a milestone at merge time (just like in the old days when we used Travis CI).

I am still trying to figure out if that workflow is subscribed to all the events that would cover all the different activity paths a PR might go through. So if you notice that job passing or failing when it is not supposed to, please let me know.

Thank you for your patience,

Pey-Lian