update the "wrapper-validation" action to v6

Author: stephengold

.github/workflows/push.yml

@@ -14,7 +14,7 @@ jobs:
           distribution: 'zulu'
           java-version: 21
       - uses: actions/checkout@v6
-      - uses: gradle/actions/wrapper-validation@v5
+      - uses: gradle/actions/wrapper-validation@v6
       - run: ./gradlew build javadoc --console=plain --stacktrace
 
   Java25-Windows: