RFR: Update README based on feedback

Tue Feb 20 15:58:04 UTC 2024

On Tue, 20 Feb 2024 15:38:01 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

> - Change references from jdk 21 to jdk 22
> - Update JEP number and recommend LLVM version
> - Amend instructions for jdk version needed to run gradle
> - Add note that example indicating that a native library is required

Looks great!

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Marked as reviewed by mcimadamore (Reviewer).

PR Review: https://git.openjdk.org/jextract/pull/222#pullrequestreview-1890820400