Update and pin gradle wrapper validation action (elastic#106635)

This commit updates the Gradle wrapper validation action to the latest
release and pins it to that version by hash per our company best
practices for GitHub Workflows & Actions.

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

@@ -10,4 +10,4 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
-      - uses: gradle/wrapper-validation-action@v1
+      - uses: gradle/wrapper-validation-action@699bb18358f12c5b78b37bb0111d3a0e2276e0e2 # Release v2.1.1