RFR: 8262236: Configure Gradle checksum verification

[John Neffenger]

On Tue, 23 Feb 2021 17:48:08 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

> > We might also consider adding the [Gradle Wrapper Validation](https://github.com/marketplace/actions/gradle-wrapper-validation) GitHub Action to the OpenJFX repository.
> 
> Feel free to file a bug and create a PR, if you are interested. I agree that this sounds like a good idea.

Isn't this configured directly through GitHub rather than with a pull request? Once that GitHub Action is added, I was thinking of following up with a pull request that upgrades the Gradle Wrapper to version 6.3. The older wrapper is probably fine, but I think we should keep the Wrapper at the same version as the distribution it downloads.

For example, when I followed the instructions for [Verifying the integrity of the Gradle Wrapper JAR](https://docs.gradle.org/current/userguide/gradle_wrapper.html#wrapper_checksum_verification) in the OpenJFX repository, it failed. Only after a brief panic did I reverse the check and discovered that we're still using the older Gradle 4.8 wrapper.

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PR: https://git.openjdk.java.net/jfx/pull/411