switch to java 21

Author: stefanseifert

.github/workflows/maven-deploy.yml

@@ -28,7 +28,7 @@ jobs:
         uses: actions/setup-java@v5
         with:
           distribution: temurin
-          java-version: 17
+          java-version: 21
           cache: maven
 
       - name: Build, verify, deploy, generate site