sync Coq policy wording with std++

README.md

@@ -33,7 +33,8 @@ This version is known to compile with:
  - Coq 8.15.2 / 8.16.1 / 8.17.0
  - A development version of [std++](https://gitlab.mpi-sws.org/iris/stdpp)
 
-Generally we always aim to support at least the last two 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.
 
 If you need to work with older versions of Coq, you can check out the
 [tags](https://gitlab.mpi-sws.org/iris/iris/-/tags) for old Iris releases that