update GitHub action for Gradle wrapper validation to v6

Author: thestinger

.github/workflows/validate-gradle-wrapper.yml

@@ -8,4 +8,4 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v6
-      - uses: gradle/actions/wrapper-validation@v5
+      - uses: gradle/actions/wrapper-validation@v6