update github job to 21

Author: R-Sandor

.github/workflows/build.yml

@@ -19,10 +19,10 @@ jobs:
           username: ${{github.actor}}
           password: ${{secrets.GITHUB_TOKEN}}
 
-      - name: Setup Java 17
+      - name: Setup Java 21
         uses: actions/setup-java@v4
         with:
-          java-version: "17"
+          java-version: "21"
           distribution: "temurin"
 
       - name: Build and Push Server Container