Locking the 3.12 branch in preparation of rc1

I’m locking the 3.12 branch (for everyone but RMs/admins) in preparation for the rc1 release. It’ll remained locked until the final 3.12.0 release. Please send any changes you want to see in 3.12.0 my way for merging.

(There’s still a couple of bugs being sorted out for rc1, so it will probably be delayed a bit more.)