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

[Zhao Song]

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.

Do we need to update README as well?

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PR Comment: https://git.openjdk.org/skara/pull/1590#issuecomment-1832866690