chore: provision toolchains with the foojay resolver

[rasros]

Summary

The foojay toolchain resolver is applied in settings, so Gradle can provision the JDK the
conventions ask for instead of requiring it to be installed already.

Why

kbuild sets jvmToolchain(25), so building needs a JDK 25 toolchain. Without a resolver a machine
that lacks one stops at:

Cannot find a Java installation on your machine matching: {languageVersion=25, ...}.
Toolchain download repositories have not been configured.

This only became worth doing with kbuild 1.2.10. Before it, the blocker was the Gradle daemon
JVM, which no resolver can provision; 1.2.10 emits Java 21 bytecode so the daemon floor is 21 again,
leaving the toolchain as the only requirement, and a toolchain can be downloaded.

It changes nothing where a JDK 25 is already present: Gradle prefers what it detects and only
downloads when nothing matches.

Testing

Verified against the case it is meant to fix, a machine with no JDK 25 in scope. Running on JDK 22
with an empty GRADLE_USER_HOME, jvmTest passes and Temurin 25.0.1 is provisioned into that
home rather than found:

<gradle-home>/jdks/eclipse_adoptium-25-amd64-linux.2  ->  Language Version: 25

Trade-off

Builds gain the ability to download JDKs, which some locked-down environments disallow. The
alternative is every developer installing JDK 25 by hand.