Add migration guide for moving the gradle files

[rekire]

Related to Resolve resolve native Flutter dependencies in Android Studio and in preparation of Extend template to resolve Dart and Kotlin to explain how to use that fix to resolve Can't resolve symbol io.flutter.plugin dependency

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[sfshaza2]

Just a bit of cleanup. Thanks, @rekire!

[sfshaza2]

/gcbrun

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 646feeb):

https://flutter-docs-prod--pr12203-patch-1-9ue7qf89.web.app