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 +11,23 @@ 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).
+
+To verify whether your app applies KGP,
+find the `kotlin-android` plugin
+(or the `org.jetbrains.kotlin.android` plugin).
+It is likely located in the
+`<app-src>/android/app/build.gradle` or
+`<app-src>/android/app/build.gradle.kts` file.
+To view the KGP application code,
+see [Update the Gradle file](#update-the-gradle-file).
+
+If your app 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][].
@@ -299,6 +316,8 @@ kotlin {
 Before enabling built-in Kotlin,
 confirm that you have migrated your application
 and any Flutter plugins it uses.
+Also, confirm that you updated your app
+to AGP 9+, because built-in Kotlin requires AGP 9+.
 
 To enable built-in Kotlin,
 set the `android.builtInKotlin` property to `true`

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

@@ -6,6 +6,23 @@ 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).
+
+To verify whether your plugin applies KGP,
+find the `kotlin-android` plugin
+(or the `org.jetbrains.kotlin.android` plugin).
+It is likely located in the
+`<plugin-project>/build.gradle` or
+`<plugin-project>/build.gradle.kts` file.
+To view the KGP application code,
+see [Update the Gradle file](#update-the-gradle-file).
+
+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][].
@@ -422,6 +439,8 @@ 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.
+Also, confirm that you updated your plugin example app
+to AGP 9+, because built-in Kotlin requires AGP 9+.
 
 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,