RFR: 7903928: update build.gradle to use JDK 23

[Nizar Benalla]

On Tue, 21 Jan 2025 18:53:33 GMT, Nizar Benalla <nbenalla at openjdk.org> wrote:

> We've updated native makefile to use JDK 23 (https://github.com/openjdk/jextract/commit/0b89a462146940a2ec90e34eeb5df3d46d8980d2), build.gradle works & expects jdk 23, it still refers to JDK var as "jdk22_home" and passes --release to be 22. Also jextract version is "22".
> 
> The mismatch has caused [some confusion](https://mail.openjdk.org/pipermail/panama-dev/2025-January/020915.html).
> 
> This patch to rename references to `jdk22_home` to `jdk_home`, the jextract version has now been bumped to `23`.
> 
> This PR has been obtained through a `git merge --squash` of #271.
> 
> TIA

README.md line 17:

> 15: ### Building
> 16: 
> 17: `jextract` depends on the [C libclang API](https://clang.llvm.org/doxygen/group__CINDEX.html). To build the jextract sources, the easiest option is to download LLVM binaries for your platform, which can be found [here](https://releases.llvm.org/download.html) (version 13.0.0 is recommended). Both the `jextract` tool and the bindings it generates depend heavily on the [Foreign Function & Memory API](https://openjdk.java.net/jeps/454), so a suitable [jdk 22+ distribution](https://jdk.java.net/23/) is also required.

This is the only change that wasn't based on review comments, I thought the download page should point to JDK 23

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PR Review Comment: https://git.openjdk.org/jextract/pull/275#discussion_r1924228123