Limit permissions to gradlew validator GH Action

I've already limited the grpc-wide setting to read-only access, but
limiting it explicitly here seems like a good idea; all workflows should
explicitly set their permissions since any action can implicitly access
the GITHUB_TOKEN.

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

@@ -1,6 +1,9 @@
 name: "Validate Gradle Wrapper"
 on: [push, pull_request]
 
+permissions:
+  contents: read
+
 jobs:
   validation:
     name: "Gradle wrapper validation"