Migrate plugin to Flutter built-in Kotlin support

[aiviapp]

Flutter 3.44.4 reports that appsflyer_sdk 6.18.0 still applies the legacy Kotlin Gradle Plugin (KGP), which future Flutter versions will no longer support.

The current android/build.gradle unconditionally applies org.jetbrains.kotlin.android and configures kotlinOptions. Please migrate the plugin to AGP 9 built-in Kotlin support following Flutter's plugin-author guide:

https://docs.flutter.dev/release/breaking-changes/migrate-to-built-in-kotlin/for-plugin-authors

Observed warning:

Your app uses the following plugins that apply Kotlin Gradle Plugin (KGP): appsflyer_sdk

[github-actions]

👋 Hi @aiviapp and Thank you for opening this issue.
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To do so, please follow this article.

[lelabo-m]

#453 already reference this, but as you can see, we don't have any news from them yet.
If you are in contact with the support, you can always check with them and comeback with the news here.