RFR: Update jextract/panama branch to track JDK 22

[Jorn Vernee]

On Thu, 15 Jun 2023 09:47:30 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> 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

Marked as reviewed by jvernee (Committer).

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PR Review: https://git.openjdk.org/jextract/pull/123#pullrequestreview-1481388993