update to Java 21 for CI

Author: thestinger

.github/workflows/build.yml

@@ -11,11 +11,11 @@ jobs:
       - uses: actions/setup-node@v3
         with:
           cache: npm
-      - name: Set up JDK 20
+      - name: Set up JDK 21
         uses: actions/setup-java@v3
         with:
           distribution: 'temurin'
-          java-version: 20
+          java-version: 21
           cache: gradle
 
       - name: Build with Gradle