Try Java 21 for builds.

.github/workflows/main.yml

@@ -33,10 +33,10 @@ jobs:
     steps:
     - name: Checkout
       uses: actions/checkout@v3
-    - name: Set up JDK 1.8
+    - name: Set up JDK 21
       uses: actions/setup-java@v3
       with:
-        java-version: 8
+        java-version: 21
         distribution: temurin
     - name: Set up PHP
       run: |