drop support for Coq 8.10

The successor, Coq 8.11, has been released in Jan 2020, almost 2 years ago. I think it is fine for us to drop earlier versions of Coq. Iris has also dropped support for 8.10 quite a while ago already.

This will also let us use https://github.com/coq/coq/issues/7910, which should help make telescopes more ergonomic (see iris!762 (merged)).