drop support for Coq 8.10
I propose we drop support for Coq 8.10. This is in accordance with out Coq support policy, which I don't think we have ever written down, but with so far has been:
- We always support at least the last two major stable Coq releases.
- We only drop support for a Coq version if the latest Iris release already supports this version.
Dropping Coq 8.10 is useful because it unblocks the persistent points-to work, and it means we can think about integrating the iris-string-ident "plugin".
I asked on Mattermost a few days ago, and so far nobody spoke up that they still require this support. I just asked again; let's see if anyone speaks up this time. @iris-users if you are using Iris with Coq 8.10 and are tracking Iris master, this will break your code. Please speak up if that affects you!
Fixes #388 (closed)