Added warning against migration to built-in kotlin

[jesswrd]

Added warning against migrating to built-in kotlin if project does not currently use KGP.

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[gemini-code-assist]

Code Review

This pull request adds warning callouts to the Kotlin migration documentation to clarify that the migration is only relevant for projects already using the Kotlin Gradle Plugin (KGP). The reviewer suggested rephrasing these warnings across all three files to specify that the migration itself is what only applies to KGP-using projects, rather than the entire guide, which resolves contradictions and ensures accuracy.

[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit 2026164):

https://flutter-docs-prod--docs-pr13634-migration-warning-1du7aw2r.web.app

[flutter-website-bot]

Staged preview of the updated flutter.dev site (updated for commit 2026164):

https://flutter-dev-230821--www-pr13634-migration-warning-ug8ucv7l.web.app

[liamappelbe]

Can the warning also mention what it means to "apply KGP"? Something like "If your project doesn't currently apply KGP (ie your gradle file doesn't have a line like apply plugin: 'kotlin-android'), ...".