Simplify kts

[Goooler]

Follow up PR for #6959, to use extensions and replace deprecated BundleTaskConvention.

/**
 * BundleTaskConvention for Gradle.
 *
 * @deprecated Replaced by BundleTaskExtension.
 */
@Deprecated
public class BundleTaskConvention

copy the constructor from

private static BundleTaskExtension getBundleTaskExtension(org.gradle.api.tasks.bundling.Jar task) {
    BundleTaskExtension extension = task.getExtensions()
	.findByType(BundleTaskExtension.class);
    if (extension != null) {
	return extension;
    }
    return task.getExtensions()
	.create(BundleTaskExtension.NAME, BundleTaskExtension.class, task);
}