RFR: Update jextract 21 with latest fixes

[Maurizio Cimadamore]

On Mon, 20 Nov 2023 18:29:19 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> This PR brings three changesets to the `master` branch, so that we can release updated binaries:
> 
> * https://git.openjdk.org/jextract/pull/132
> * https://git.openjdk.org/jextract/pull/131
> * https://github.com/openjdk/jextract/pull/137

src/main/java/org/openjdk/jextract/impl/ClassSourceBuilder.java line 216:

> 214:         }
> 215:         indent();
> 216:         append(" * {@snippet lang=c :\n");

This is a new fix - see: https://github.com/openjdk/jextract/pull/140

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PR Review Comment: https://git.openjdk.org/jextract/pull/139#discussion_r1399614433