mention Coq 8.11 in changelog

CHANGELOG.md

@@ -20,7 +20,7 @@ Coq development, but not every API-breaking change is listed.  Changes marked
 
 **Changes in Coq:**
 
-* Added support for Coq 8.10; dropped support for Coq 8.7.
+* Added support for Coq 8.10 and Coq 8.11; dropped support for Coq 8.7.
 * A new tactic `iStopProof` to turn the proof mode entailment into an ordinary
   Coq goal `big star of context ⊢ proof mode goal`.
 * Rename `iProp`/`iPreProp` to `iPropO`/`iPrePropO` since they are `ofeT`s.