This drops support for Coq 8.9. That version of Coq is ancient, and is preventing us from following some deprecations of Coq 8.10. It also increases the risk of accidental CI breakage due to version incompatibilities. Coq 8.9 is simply not worth the effort any more.
If someone really needs Coq 8.9, they can use Iris 3.3.
Fixes #318 (closed)