docs: added kotlin version step in flutter guide

Author: saksham-gt

docs/partials/_flutter-quickstart.mdx

@@ -50,6 +50,12 @@ flutter pub add dyte_uikit
       }
   ```
 
+And change the kotlin version to `1.9.0`
+
+```kotlin
+    ext.kotlin_version = '1.9.0'
+```
+
 - ### **iOS**
 
   1. Set minimum deployment target for your Flutter app to 13.0 or higher.