ANN: astropy v6.1 will not support Python 3.9

[Pey Lian Lim]

1. I merged https://github.com/astropy/astropy/pull/15603 that drops support for Python 3.9. This means astropy v6.1 will only be for Python 3.10 or later.

2. I will update the main branch protection rules that go with this, so any existing open PRs will have to be rebased to pass the CI check. (But not yet, give me like another 30 mins before rebasing...)

FYI.