Add bot running tests on JDK 21

Change-Id: I9edad00627bbd3cec20511e8498cb91980730edb
diff --git a/tools/test.py b/tools/test.py
index efbe5a1..dcff8c9 100755
--- a/tools/test.py
+++ b/tools/test.py
@@ -51,7 +51,7 @@
     'jdk9',
     'jdk11',
     'jdk17',
-    'jdk20',
+    'jdk21',
 ] + ['dex-%s' % dexvm for dexvm in ALL_ART_VMS]