Upgrade SonarCloud workflow to use JDK 21

Author: stefanhahmann

.github/workflows/sonarcloud.yml

@@ -39,10 +39,10 @@ jobs:
 
     steps:
       - uses: actions/checkout@v2
-      - name: Set up JDK 17
+      - name: Set up JDK 21 (for Sonar scanning)
         uses: actions/setup-java@v4
         with:
-          java-version: '17'
+          java-version: '21'
           distribution: 'zulu'
           cache: 'maven'
       - name: Jacoco Report and SonarCloud Analysis