Drop support for Coq 8.14.

Coq 8.17 has been released, https://github.com/coq/coq/releases/tag/V8.17.0

In !440 (merged) we decided to support minimal 3 stable Coq versions, so that's 8.17, 8.16, and 8.15.

This means we can finally merge !437 (closed).

TODO (for @jung): The CI file still means 8.17.dev, that probably should be turned into the stable 8.17 release and we should add 8.18.dev instead.

mentioned in merge request iris!908 (merged)

We first need to drop 8.14 from Iris: iris!908 (merged)

added 15 commits

• 05ffa78b...ffe033d2 - 14 commits from branch master
• e0233179 - Drop support for Coq 8.14.

Compare with previous version

I have rebased this MR. @jung Can you check it everything regarding the CI and opam files is OK, and then merge?

merged

mentioned in commit e29ac56f