RFR: 2112: Update JDK to 21 and Gradle to 8.4 [v3]

[Kevin Rushforth]

On Thu, 30 Nov 2023 00:17:32 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.
>
> Erik Joelsson has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Spelling

Looks good.

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Marked as reviewed by kcr (Reviewer).

PR Review: https://git.openjdk.org/skara/pull/1590#pullrequestreview-1757412813