add Coq 8.21 to NOREF

Makefile.coq.local

@@ -29,7 +29,7 @@ COQ_TEST=$(COQTOP) $(COQDEBUG) -batch -test-mode
 # These versions of Coq are known to have different output so we don't test them.
 # Need to make this a lazy variable (`=` instead of `:=`) since COQ_VERSION is only set later.
 # Make sure to recognize both 8.$NUM.0 and 8.$NUM+alpha.
-COQ_NOREF=$(shell echo "$(COQ_VERSION)" | grep -E "^8\.(16|17|18|20)[.+]" -q && echo 1)
+COQ_NOREF=$(shell echo "$(COQ_VERSION)" | grep -E "^8\.(16|17|18|20|21)[.+]" -q && echo 1)
 
 tests/.coqdeps.d: $(TESTFILES)
 	$(SHOW)'COQDEP TESTFILES'