Fix Java 25 Build

[Kas-tle]

Jitpack parts can be reverted when Architectury is dropped.

[Copilot]

Pull request overview

This PR updates the Gradle build configuration to keep builds working on newer Java runtimes (e.g., Java 25) by pinning toolchain usage to Java 21 and adjusting plugin resolution to a forked Architectury plugin.

Changes:

• Add Foojay toolchain resolver convention plugin to enable auto-downloading toolchains.
• Enforce Java 21 toolchains for build-logic and modded convention builds.
• Switch Architectury plugin resolution to a fork/commit and add JitPack as a repository source.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
settings.gradle.kts	Adds JitPack to pluginManagement and applies the Foojay toolchain resolver convention plugin.
gradle/libs.versions.toml	Points architectury-plugin to a fork/commit-based coordinate.
build-logic/src/main/kotlin/geyser.modded-conventions.gradle.kts	Forces Java 21 toolchain for modded convention projects.
build-logic/settings.gradle.kts	Applies the Foojay toolchain resolver convention plugin for the included build.
build-logic/build.gradle.kts	Forces Java 21 toolchain for build-logic and adds JitPack to repositories.

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