update to java 21

Author: Haehnchen

.github/workflows/gradle.yml

@@ -20,10 +20,10 @@ jobs:
       with:
         submodules: recursive
 
-    - name: Set up JDK 19
-      uses: actions/setup-java@v2
+    - name: Set up JDK 21
+      uses: actions/setup-java@v4
       with:
-        java-version: '19'
+        java-version: '21'
         distribution: 'adopt'
 
     # Cache Gradle dependencies