Use Java 21 when building in CI

Author: amyavi

.github/workflows/build.yml

@@ -9,10 +9,10 @@ jobs:
         uses: actions/checkout@v4
       - name: validate gradle wrapper
         uses: gradle/wrapper-validation-action@v2
-      - name: setup jdk 17
+      - name: setup jdk 21
         uses: actions/setup-java@v4
         with:
-          java-version: 17
+          java-version: 21
           distribution: 'microsoft'
       - name: make gradle wrapper executable
         run: chmod +x ./gradlew