drop support for Coq 8.11
Currently, every second MR breaks on Coq 8.11, because typeclass inference has gotten better with Coq 8.12 in particular around inferring the domain type for a given map. To avoid having to fix things for Coq 8.11 all the time, I propose we drop support for Coq 8.11. This is in line with our usual policy of supporting the last two stable Coq releases.