Prepare for https://github.com/coq/coq/pull/19059

Prepare for next version number bump in Coq.

Could you please encode the test in the other direction to avoid having to do that kind of PR again for next Coq version (no hurry, this can be done in the next six months).

Makefile.coq.local

@@ -27,7 +27,7 @@ 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'