RFR: 7903500: Add Gradle Wrapper Validation workflow to jtreg [v2]

[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

Christian Stein has updated the pull request incrementally with one additional commit since the last revision:

  Update gradle-wrapper-validation.yml
  
  Fine-tune event triggers

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Changes:
  - all: https://git.openjdk.org/jtreg/pull/159/files
  - new: https://git.openjdk.org/jtreg/pull/159/files/36fa597f..67807a0d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jtreg&pr=159&range=01
 - incr: https://webrevs.openjdk.org/?repo=jtreg&pr=159&range=00-01

  Stats: 5 lines in 1 file changed: 4 ins; 0 del; 1 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