Added warning against migration to built-in kotlin

sites/docs/src/content/release/breaking-changes/migrate-to-built-in-kotlin/for-app-developers.md

@@ -11,6 +11,13 @@ an AGP version created before 9.0.0 to an AGP version 9.0.0+.
 You should also use the minimum compatible dependency versions
 listed in the [Android Gradle Plugin docs][AGP block].
 
+:::warning
+This guide only applies to apps that already use the
+Kotlin Gradle Plugin (KGP).
+If your project doesn't currently apply KGP,
+don't migrate to built-in Kotlin.
+:::
+
 :::note
 To update Flutter plugins to use built-in Kotlin,
 follow the [migration guide for plugin authors][].

sites/docs/src/content/release/breaking-changes/migrate-to-built-in-kotlin/for-plugin-authors.md

@@ -6,6 +6,13 @@ description: >-
 
 This guide outlines the migration steps specifically for plugin authors.
 
+:::warning
+This guide only applies to plugins that already use the
+Kotlin Gradle Plugin (KGP).
+If your plugin project doesn't currently apply KGP,
+don't migrate to built-in Kotlin.
+:::
+
 :::note
 To update Flutter apps to use built-in Kotlin,
 follow the [migration guide for app developers][].
@@ -421,7 +428,8 @@ the newly released plugin version:
 
 Before enabling built-in Kotlin,
 confirm that you have migrated your plugin example app
-and any Flutter plugins it uses.
+and any Flutter plugins it uses. Also confirm you have updated your
+plugin example app to AGP version 9.0.0+.
 
 To enable built-in Kotlin,
 set the `android.builtInKotlin` property to `true`

sites/docs/src/content/release/breaking-changes/migrate-to-built-in-kotlin/index.md

@@ -7,6 +7,13 @@ description: >-
 
 {% render "docs/breaking-changes.md" %}
 
+:::warning
+This guide only applies to apps and plugins that already use the
+Kotlin Gradle Plugin (KGP).
+If your project doesn't currently apply KGP,
+don't migrate to built-in Kotlin.
+:::
+
 ## Summary
 
 To build a Flutter app for Android,