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.