Setup matrix so multiple java versions are tested.

.github/workflows/build_validator.yml

@@ -15,15 +15,18 @@ on:
 
 jobs:
   build:
+    strategy:
+      matrix:
+        jdk: [8, 11, 14, 16, 17]
 
     runs-on: ubuntu-latest
 
     steps:
     - uses: actions/checkout@v2
-    - name: Set up JDK 11
+    - name: Set up JDK ${{ matrix.jdk }}
       uses: actions/setup-java@v1
       with:
-        java-version: '11'
+        java-version: ${{ matrix.jdk }}
         distribution: 'adopt'
         cache: gradle
     - name: Validate Gradle wrapper