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

[Zhao Song]

On Thu, 30 Nov 2023 13:37:14 GMT, Erik Joelsson <erikj at openjdk.org> wrote:

> > The Gradle 8.5 release notes has a section on "[Full Java 21 Support](https://docs.gradle.org/8.5/release-notes.html#full-java-21-support)".
> 
> Wow, they released yesterday according to the page. When I checked yesterday it was still Gradle 8.4 listed as latest. I will update the patch to use 8.5 instead so we have official support.

BTW, yesterday, I updated gradle in my Mac with homebrew and homebrew updated it to 8.5

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PR Comment: https://git.openjdk.org/skara/pull/1590#issuecomment-1834130191