Upgrade build environment to Java 11

Author: minoneer

.github/workflows/maven.yml

@@ -16,10 +16,10 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
-    - name: Set up JDK 1.8
-      uses: actions/setup-java@v1
+    - uses: actions/setup-java@v2
       with:
-        java-version: 1.8
+        distribution: 'adopt'
+        java-version: '11'
     - name: Build with Maven
       run: mvn -B package
     - name: Upload plugin artifact