Merge branch 'ralf/coqsupport' into 'master'

Coq support policy: we support 2 Coq versions

See merge request !480

README.md

@@ -45,7 +45,8 @@ This version is known to compile with:
 
  - Coq version 8.15.2 / 8.16.1 / 8.17.0
 
-Generally we always aim to support at least the last three stable Coq releases.
+Generally we always aim to support the last two stable Coq releases. Support for
+older versions will be dropped when it is convenient.
 
 ## Installing via opam