Support AGP 9 built-in Kotlin

[SLIPPECAT]

Summary

Android Gradle Plugin 9.0 ships built-in Kotlin support and registers its own kotlin { } extension. Unconditionally applying apply plugin: 'kotlin-android' on top of it now fails with:

Failed to apply plugin 'kotlin-android'.
> ⛔ Failed to apply plugin 'org.jetbrains.kotlin.android'
  The 'org.jetbrains.kotlin.android' plugin is no longer required for Kotlin support since AGP 9.0.

This currently blocks any app that has adopted AGP 9 with built-in Kotlin enabled (the default once on AGP 9, unless the app opts out) from depending on flutter_naver_map. Flutter's own build now flags this plugin explicitly:

WARNING: Your app uses the following plugins that apply Kotlin Gradle Plugin (KGP): ... flutter_naver_map ...
Future versions of Flutter will fail to build if your app uses plugins that apply KGP.

Fix

Per Flutter's Built-in Kotlin migration guide for plugin authors:

• Only apply 'kotlin-android' when it's actually needed: on AGP < 9, or on AGP 9+ where the consuming app has explicitly opted out via android.builtInKotlin=false in gradle.properties (this is currently Flutter's recommended interim state for most AGP 9 adopters, since much of the plugin ecosystem hasn't migrated yet).
• Move the jvmTarget configuration out of the deprecated android { kotlinOptions { } } block into the top-level kotlin { compilerOptions { } } block, since that extension is registered in both the classic-plugin and built-in-Kotlin cases.

Testing

Verified against a real Flutter app (AGP 9.0.1, Gradle 9.1.0, android.builtInKotlin=false — today's supported "opt-out" combination) via a local path dependency_override: flutter build appbundle --release succeeds with this change, producing the same output as with the currently published version — no regression for existing users on this combination or on AGP < 9.

The android.builtInKotlin=true path (requires Flutter 3.47+) follows AGP's documented built-in-Kotlin behavior, but I wasn't able to independently build-test it since it needs a newer Flutter SDK than what's available in my test app.

Happy to adjust the approach if you'd prefer a different structure (e.g. matching the pattern used in flutter/packages' first-party plugins).

[SLIPPECAT]

Friendly bump on this — Flutter 3.47.2 (stable) is out now, which is the SDK version needed to actually build-test the android.builtInKotlin=true path mentioned above (the one part I couldn't verify when I opened this PR). I've upgraded my test app to 3.47.2 — happy to run that verification and post the results here if it'd help move this forward. Let me know if there's anything else needed on my end.