RFR: 7903500: Add Gradle Wrapper Validation workflow to jtreg

[Christian Stein]

This change adds a workflow to this repository to verify that any and all `gradle-wrapper.jar` files in the repository match the SHA-256 checksums of any of our official releases. See details at https://github.com/marketplace/actions/gradle-wrapper-validation

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Commit messages:
 - Create gradle-wrapper-validation.yml

Changes: https://git.openjdk.org/jtreg/pull/159/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=159&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903500
  Stats: 10 lines in 1 file changed: 10 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jtreg/pull/159.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/159/head:pull/159

PR: https://git.openjdk.org/jtreg/pull/159