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.
Merge request reports
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Blocked on iris!882 (merged)
As I mentioned to @jung in person at POPL, I am in favor of this (both for std++ and Iris).
It seems there were no objections at Mattermost either, so feel free to merge.
enabled an automatic merge when the pipeline for 84bb9872 succeeds
mentioned in commit 7125e166
mentioned in merge request !468 (merged)
