Add gradle wrapper validation

Gradle publishes this workflow to allow validation of gradle wrapper JARs to ensure that they actually are the jars published by Gradle. See: https://github.com/gradle/wrapper-validation-action#add-to-an-existing-workflow

.github/workflows/main.yml

@@ -20,6 +20,7 @@ jobs:
       - uses: actions/checkout@v2
         with:
           submodules: recursive
+      - uses: gradle/wrapper-validation-action@v1
       - name: Use java ${{ matrix.java }}
         uses: actions/setup-java@v3
         with: