drop support for Coq 8.12 and 8.13

Dropping 8.12 is needed for Zify.zify_pre_hook in !439.

I also propose we document an official support policy of supporting always at least the last 3 stable Coq releases. Looking at how we dropped old Coq versions in the past, I think this was always true. Most releases of std++ supported 4 or even 5 Coq versions, but 1.2.0 and 1.2.1 only supported 3 versions. However I feel like more than 3 releases shouldn't really be needed: right now this means the oldest Coq we support, Coq 8.14, is more than 14 months old.

Because we are dropping Coq versions anyway, and to test at least with 1 sample the impact of this policy, I suggest we also drop Coq 8.13, so we are down to the minimal set of 3 supported Coq versions.