RFR: Update jextract/panama branch to track JDK 22

[Maurizio Cimadamore]

This PR contains some initial changes to enable jextract support for the latest Panama repo (and JDK 22 in general).

It contains the usual updates to source, build and readme files (e.g. to replace jdk21 with jdk22).

On top of that there are some changes to support the latest API changes in the Panama repo such as:
* removal of implicit sequence layout factory
* renaming of XYZUtf8String -> XYZString

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/jextract/pull/123/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=123&range=00
  Stats: 39 lines in 19 files changed: 1 ins; 0 del; 38 mod
  Patch: https://git.openjdk.org/jextract/pull/123.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/123/head:pull/123

PR: https://git.openjdk.org/jextract/pull/123