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

[Erik Joelsson]

On Wed, 29 Nov 2023 22:40:42 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

> 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.

Oh, I saw that before, but didn't remember which version it was. When I looked again recently, the initial formulation fooled me:

> [Java 21](https://docs.gradle.org/8.4/release-notes.html#java-21) is now supported for compiling, testing, and running such projects.

But further down I see you are right:

> Currently, you cannot run Gradle on Java 21 because Kotlin lacks support for JDK 21. However, support for running Gradle with Java 21 is expected in future versions.

It does appear to work fine however. Our Gradle scripts aren't using Kotlin at all, it's still all Groovy. I would have expected Gradle to protest if it didn't work.

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PR Comment: https://git.openjdk.org/skara/pull/1590#issuecomment-1832852943