Update CI to Java 21

Author: SuperGamerTron

.github/workflows/ci.yml

@@ -34,7 +34,7 @@ jobs:
     - name: Setup Java JDK
       uses: actions/setup-java@v1
       with:
-        java-version: 17
+        java-version: 21
         java-package: jre
     - name: Install xi
       run: sudo apt-get install -y libxi-dev libglu1-mesa-dev