add tests for java 21

Author: TheSnoozer

.github/workflows/default-tests.yml

@@ -31,7 +31,7 @@ jobs:
     needs: checkstyle
     strategy:
       matrix:
-        java_version: ['11', '12', '13', '14', '15', '16', '17', '18', '19', '20']
+        java_version: ['11', '12', '13', '14', '15', '16', '17', '18', '19', '20', '21']
 
     steps:
       - uses: actions/checkout@v4