Configure Gradle daemon JVM toolchain

[koperagen]

Summary

• add the generated gradle/gradle-daemon-jvm.properties file
• require Java 21 for the Gradle daemon, matching the existing gradle-jdk version
• provide Foojay download URLs for supported x86-64 and AArch64 platforms so Gradle can provision the daemon JDK when no compatible local JDK is installed

Validation

• confirmed the root settings convention already applies org.gradle.toolchains.foojay-resolver-convention 1.0.0
• confirmed the project pins the Gradle JDK to Java 21
• verified the branch is one commit ahead of master and changes only the generated daemon JVM properties file

Full build not run; this is a generated build-configuration-only change.

Closes #2016

[koperagen]

Updated the implementation after review of the maintenance workflow.

The generated properties file is no longer the only change. The root updateDaemonJvm task now reads its language version from libs.versions.gradle.jdk, making that catalog entry the single source of truth for both project toolchains and the Gradle daemon.

Upgrade workflow, including a future move from JDK 21 to JDK 25:

1. Change gradle-jdk in gradle/libs.versions.toml.
2. Run ./gradlew updateDaemonJvm.
3. Commit the regenerated gradle/gradle-daemon-jvm.properties.

The literal platform URLs in that file are generated task output, analogous to Gradle Wrapper metadata; they are not intended to be edited manually. CONTRIBUTING.md now documents the same workflow and no longer lists a locally installed JDK 21 as a prerequisite.

This comment supersedes the original PR summary, which described the initial generated-file-only version of the change.