Fix Gradle 9 deprecations

[demoth]

Summary

• switch to java plugin extension instead of deprecated JavaPluginConvention
• stop using mainClassName property and update manifest/native image config

Testing

• ./gradlew help --warning-mode all

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https://chatgpt.com/codex/tasks/task_e_6848a11b54208329ad9e06152981c463

[Copilot]

Pull Request Overview

A pull request to address Gradle 9 deprecations by switching to a supported Java plugin extension and updating the configuration for the main class in both native image configuration and the application manifest.

• Updated nativeimage.gradle to use mainClass.set() with a hardcoded value.
• Revised build.gradle to remove deprecated mainClassName property and update the application configuration and manifest accordingly.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
cake/lwjgl3/nativeimage.gradle	Updated mainClass assignment to use the .set() method with a hardcoded launcher name.
cake/lwjgl3/build.gradle	Converted mainClass configuration to use a variable and updated the manifest attributes accordingly.

[Copilot]

[nitpick] Consider externalizing the main class name to a shared configuration or variable to ensure consistency with the build.gradle settings, which can help reduce duplication across files.

Suggested change

	mainClass.set('org.demoth.cake.lwjgl3.Lwjgl3GameLauncher')
	mainClass.set(mainClassName)