fix: support AGP 9 built-in Kotlin

[hatemragab]

Summary

• stop applying the Kotlin Gradle Plugin from the Android library
• migrate JVM target configuration to kotlin.compilerOptions
• raise the supported toolchain to Flutter 3.44 / Dart 3.12
• validate both legacy KGP compatibility and AGP 9 built-in Kotlin in CI

Fixes #25

Compatibility impact

• existing apps can continue using Flutter's temporary KGP compatibility mode
• apps enabling AGP 9 built-in Kotlin require Flutter 3.47 or newer
• no method-channel or runtime API changes

Validation

• flutter analyze — passed
• flutter test — 97 tests passed
• cd example && flutter build apk --debug — passed
• cd example/android && ./gradlew :v_video_compressor:testDebugUnitTest lintDebug — passed
• FLUTTER_ANDROID_SKIP_BUILD_DEPENDENCY_VALIDATION=true tool/verify_android_built_in_kotlin.sh — passed with Flutter 3.47.4
• git diff --check and workflow/shell syntax checks — passed

The dependency-validation bypass is limited to the isolated built-in Kotlin fixture and documents flutter/flutter#192167.