Validate Java Gradle wrappers in workflow examples

[axieum]

What article on docs.github.com is affected?

Building and testing Java with Gradle

What part(s) of the article would you like to see updated?

Gradle wrappers should always be validated and hence the use of the official Gradle Wrapper Validation Action should be added.

- name: Validate Gradle wrapper
  uses: gradle/wrapper-validation-action@v1

Additional information

None.

[welcome]

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