Use Java 11 for building

Author: Andre601

.github/workflows/build_docs.yml

@@ -26,11 +26,10 @@ jobs:
       run: |
         mkdocs build
     # Build Javadoc
-    - uses: actions/checkout@v2
-    - name: Setup Java 8
+    - name: Setup Java 11
       uses: actions/setup-java@v1
       with:
-        java-version: 1.8
+        java-version: '11.0.4'
     - name: Build Javadoc
       run: |
         ./gradlew javadoc