Bump GitHub CI to J21

Author: Aeltumn

.github/workflows/build.yml

@@ -19,7 +19,7 @@ jobs:
             - name: Setup Java JDK
               uses: actions/setup-java@v4
               with:
-                  java-version: 17
+                  java-version: 21
                   distribution: temurin
 
             - name: Gradle Wrapper Validation