RFR: Update README based on feedback

[Jorn Vernee]

- 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

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Commit messages:
 - Update README.md
 - Update README or comments

Changes: https://git.openjdk.org/jextract/pull/222/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=222&range=00
  Stats: 9 lines in 1 file changed: 2 ins; 1 del; 6 mod
  Patch: https://git.openjdk.org/jextract/pull/222.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/222/head:pull/222

PR: https://git.openjdk.org/jextract/pull/222