test(ci): Java 25 for all jobs

[c-dilks]

don't merge

[c-dilks]

cool, this reproduces what I see locally (with Java 25, Gradle 9.2.0); error from this CI run is:

Welcome to Gradle 8.14.1!

Here are the highlights of this release:
 - Java 24 support
 - GraalVM Native Image toolchain selection
 - Enhancements to test reporting
 - Build Authoring improvements

For more details see https://docs.gradle.org/8.14.1/release-notes.html

Starting a Gradle Daemon (subsequent builds will be faster)

FAILURE: Build failed with an exception.

* What went wrong:
BUG! exception in phase 'semantic analysis' in source unit '_BuildScript_' Unsupported class file major version 69
> Unsupported class file major version 69

* Try:
> Run with --stacktrace option to get the stack trace.
> Run with --info or --debug option to get more log output.
> Run with --scan to get full insights.
> Get more help at https://help.gradle.org./

BUILD FAILED in 8s

See temporary build directory for more info:
	/home/runner/work/coatjava/coatjava/tmp.install-clara.znXhsB

[c-dilks]

confirmed locally that:

• building clara-java with Java 25 fails, since set Gradle version 8 is too old
• Gradle 9 introduced changes which require updates to build.gradle