RFR: Upgrade to Gradle 8.0.2

[James Yuzawa]

Upgrade to the most recent gradle version. The prior version (7.3.3) did not support Java versions after 17 per https://docs.gradle.org/current/userguide/compatibility.html This change will allow user to build jextract on Java 19. I had to make sure I had a JDK 17 around for the gradle, set the JAVA_HOME, then make sure I had another proper JDK to bootstrap the build. Additionally, I added a gradle wrapper validation github action to ensure the integrity of the wrapper.

Question: is "master" not longer the default branch in this repo? Should these changes to into master and then ported to the various versions branches? Let me know how you may want to proceed.

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Commit messages:
 - Upgrade to Gradle 8.0.2

Changes: https://git.openjdk.org/jextract/pull/110/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=110&range=00
  Stats: 29 lines in 5 files changed: 18 ins; 1 del; 10 mod
  Patch: https://git.openjdk.org/jextract/pull/110.diff
  Fetch: git fetch https://git.openjdk.org/jextract pull/110/head:pull/110

PR: https://git.openjdk.org/jextract/pull/110