RFR: 2112: Update JDK to 21 and Gradle to 8.4

[Kevin Rushforth]

On Wed, 29 Nov 2023 22:33:25 GMT, Erik Joelsson <erikj at openjdk.org> wrote:

> Bumping JDK version to 21.0.1 and Gradle version to 8.4.
> 
> There were two tests that needed to be updated. One was assuming iteration order on HashMap. The other was more complex as it was triggered by [JDK-8289689](https://bugs.openjdk.org/browse/JDK-8289689). My conclusion is that we can't count on unicode normalization returned from Git to match what the files in the filesystem have on mac, and so the test needs to accommodate for that. There is a longer explanation in a comment.

gradle 8.4 doesn't support running gradle itself on JDK 21. Do you use a separate JDK to run gradle (separate from the JDK 21 that you will use for building and testing Skara)? If not, you might be just getting lucky.

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PR Comment: https://git.openjdk.org/skara/pull/1590#issuecomment-1832813997