Auto-assign milestone based on base branch

This is simple but might not be feasible but @ahf asked me to open this anyway.

Essentially, if the MR opens against maint-0.4.7 then set the milestone to "0.4.7-post-stable". If it is against main, set it to 0.4.8-freeze.

We could just have a matrix that we update when we release a new stable or milestone move around.

I'm asking this because I keep needing to set the milestone to the new MRs and it is a bit of work that could be automated.