RFR: Reference latest JEP of FFM API.

[Brayan Munoz V.]

Let's reference the JEP 454 which is the latest for FFM API instead of the JEP 442 of the third preview. 

In the README.md the 454 is already being referenced.  https://github.com/openjdk/jextract/blob/b4da3284d41863af47d4a36b5829849b27cd47f4/README.md?plain=1#L3

Please add the following to the description of the pull request:

1. A brief recap of the status quo, as it relates to the subject of the pull request.
2. A description of why the status quo is problematic.
3. A description of how this pull request addresses this issue.
4. If you ran into issues while making changes in the code that you had to work around,
  please mention these as well, as this helps reviewers understand the changes that have been made.

For 1 and 2 it is also okay to refer to the JBS ticket, if that already contains a comprehensive
problem description.

Please test your pull request before submitting it by running `./gradlew jtreg`. If you're
not able to test locally on your machine, please indicate this in the pull request description,
and indicate which testing has been done instead (or indicate that no testing has been done).

It is possible to run tests through Github actions if you enable them for your fork (this is free).
Github actions can be enabled for your fork from the 'Actions' tab. The tests will then run
automatically after the pull request has been created.

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Commit messages:
 - Reference latest JEP of FFM API.

Changes: https://git.openjdk.org/jextract/pull/243/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=243&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jextract/pull/243.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/243/head:pull/243

PR: https://git.openjdk.org/jextract/pull/243