Update migration notice in README files

Revised both English and Chinese README files to announce migration to libnativeapi/nativeapi-flutter, referencing the new unified C++ core library for improved cross-platform native API support. Removed outdated migration notes and updated links accordingly.

Author: lijy91

README-ZH.md

@@ -1,4 +1,6 @@
-> **🚀 快速发布您的应用**: 试试 [Fastforge](https://fastforge.dev) - 构建、打包和分发您的 Flutter 应用最简单的方式。
+> **⚠️ 迁移通知**: 本插件正在迁移到 [libnativeapi/nativeapi-flutter](https://github.com/libnativeapi/nativeapi-flutter)
+>
+> 新版本基于统一的 C++ 核心库（[libnativeapi/nativeapi](https://github.com/libnativeapi/nativeapi)），提供更完整、一致的跨平台原生 API 支持。
 
 # window_manager
 
@@ -14,8 +16,6 @@
 
 这个插件为 Flutter 桌面应用程序提供了全面的窗口管理功能，使开发者能够完全控制窗口大小、位置、外观、关闭行为，以及监听事件。
 
-> 注意：本插件计划迁移至 [nativeapi](https://github.com/leanflutter/nativeapi-flutter) 以提升可维护性和性能，但目前该方案仍处于实验阶段。
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 [English](./README.md) | 简体中文

README.md

@@ -1,4 +1,6 @@
-> **🚀 Ship Your App Faster**: Try [Fastforge](https://fastforge.dev) - The simplest way to build, package and distribute your Flutter apps.
+> **⚠️ Migration Notice**: This plugin is being migrated to [libnativeapi/nativeapi-flutter](https://github.com/libnativeapi/nativeapi-flutter)
+>
+> The new version is based on a unified C++ core library ([libnativeapi/nativeapi](https://github.com/libnativeapi/nativeapi)), providing more complete and consistent cross-platform native API support.
 
 # window_manager
 
@@ -14,8 +16,6 @@
 
 This plugin provides comprehensive window management capabilities for Flutter desktop applications, enabling full control over window size, position, appearance, close behavior, and listening to events.
 
-> Note: This plugin is planned to migrate to [nativeapi](https://github.com/leanflutter/nativeapi-flutter) to improve maintainability and performance, but the solution is still experimental at this stage.
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 English | [简体中文](./README-ZH.md)