RFR: Gradle 8.11.1 update cleanups

[Jorn Vernee]

Some things were missed as part of: https://github.com/openjdk/jextract/pull/264

- The action to validate the gradle wrapper accidentally uses a deprecated version
- The readme was not updated.

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Commit messages:
 - update readme
 - use non-deprecated action to validate gradle wrapper

Changes: https://git.openjdk.org/jextract/pull/272/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=272&range=00
  Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jextract/pull/272.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/272/head:pull/272

PR: https://git.openjdk.org/jextract/pull/272