Update GH runner to use newer version of Java

We still target 11 for the runtime requirement.

Author: zachbr

.github/workflows/gradle.yml

@@ -12,7 +12,7 @@ jobs:
     - uses: actions/setup-java@v4
       with:
         distribution: 'temurin'
-        java-version: '11'
+        java-version: '21'
         cache: 'gradle'
     - name: Build with Gradle
       run: ./gradlew build --no-daemon