Major Refactor: Kotlin + Swift rewrite, Gradle 8 upgrade, cleanup

[vicajilau]

Descripción del PR:

This PR introduces a complete rewrite and modernization of the plain_notification plugin, with major improvements across both Android and iOS platforms:

Summary of Changes

Plugin Modernization

• Rewritten Android implementation using Kotlin 1.9
• Rewritten iOS implementation using Swift 5
• Removed all legacy Java and Objective-C code
• General cleanup of deprecated patterns and unused code

Build System Updates

• Upgraded to Gradle 8 for Android compatibility with newer tooling and JDK versions

Testing

• Fully tested on both Android and iOS real devices

Example Project

• Updated the example project to reflect the new implementation
• Verified plugin functionality from the example app

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Let me know if you'd like a changelog entry or migration notes included.