[integration tests] from now on also use jdk21

.github/workflows/ant.yml

@@ -136,7 +136,7 @@ jobs:
     needs: build
     strategy: 
       matrix: 
-        jdk: [8, 11, 17]
+        jdk: [8, 11, 17, 21]
         tool: 
           - {name: "maven",  cmd: "mvn compile"}
           - {name: "gradle", cmd: "gradle assemble"}