Fix github-actions build

Author: TBlueF

.github/workflows/gradle.yml

@@ -9,10 +9,10 @@ jobs:
     - uses: actions/checkout@v1
       with: 
         submodules: recursive
-    - name: Set up JDK 1.8
+    - name: Set up Java 11
       uses: actions/setup-java@v1
       with:
-        java-version: 1.8
+        java-version: 11
     - name: Build with Gradle
       run: ./gradlew build
     - uses: actions/upload-artifact@v2-preview