RFR: 8263204: Add Gradle Wrapper Validation Action [v4]

[Kevin Rushforth]

On Mon, 8 Mar 2021 22:47:16 GMT, John Neffenger <github.com+1413266+jgneff at openjdk.org> wrote:

>> See the [Gradle Wrapper Validation Action](https://github.com/marketplace/actions/gradle-wrapper-validation) for details on this pull request. I'll test the changes with the following sequence of commits:
>> 
>> 1. This commit adds a tampered Gradle Wrapper JAR file, which should go undetected.
>> 2. The next commit will add the Official Gradle Wrapper Validation Action, which should detect the tampered file.
>> 3. The final commit will remove the tampered file and replace it with the original Gradle 4.8 Wrapper.
>
> John Neffenger has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Restore the Gradle version 4.8 Wrapper JAR file

Marked as reviewed by kcr (Lead).

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PR: https://git.openjdk.java.net/jfx/pull/419