Raise Java Version from 17 to 21.

Author: tk

.github/workflows/build.yml

@@ -13,10 +13,10 @@ jobs:
       - uses: actions/checkout@v4
         with:
           fetch-depth: 0  # Shallow clones should be disabled for a better relevancy of analysis
-      - name: Set up JDK 17
+      - name: Set up JDK 21
         uses: actions/setup-java@v4
         with:
-          java-version: 17
+          java-version: 21
           distribution: 'zulu' # Alternative distribution options are available.
       - name: Cache SonarQube packages
         uses: actions/cache@v4