RFR: 2112: Update JDK to 21 and Gradle to 8.5 [v5]

[mrcode]

On Thu, 30 Nov 2023 22:36:15 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:
> 
>   Update readme

Unsupported Java. 
Your build is currently configured to use Java 21.0.1 and Gradle 8.5.

Possible solution:
 - Use Java 20 as Gradle JVM: Open Gradle settings 
 - Open Gradle wrapper settings, change `distributionUrl` property to use compatible Gradle version and reload the project
 
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Why can't I run my project with JDK 21 on Gradle 8.5 final version?

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PR Comment: https://git.openjdk.org/skara/pull/1590#issuecomment-1837994577