Add deprecation notice for TRTC Web SDK v4

Added deprecation notice for TRTC Web SDK v4 and recommended v5.

Author: Rychou

v4/README.md

@@ -1,3 +1,18 @@
+> ⚠️ **Deprecated Notice (Important)**
+>
+> The **TRTC Web SDK v4 (`trtc-js-sdk`) is now in maintenance mode and is no longer recommended for new projects.**
+>
+> 👉 Please use **TRTC Web SDK v5** instead:
+> - 📦 npm: https://www.npmjs.com/package/trtc-sdk-v5
+> - 📖 Documentation: https://trtc.io/document/59649
+> - 💻 GitHub: https://github.com/Tencent-RTC/TRTC_Web
+>
+> v5 introduces a modern **track-based API design**, improved performance, and better extensibility.
+>
+> ---
+> ✅ **If you are starting a new project, always choose v5.**  
+> ⚠️ v4 will only receive critical bug fixes and no new features.
+
 # TRTC Web V4 SDK
 
-checkout this repository [TRTC Web V4 SDK](https://github.com/TencentRTC/web)
+checkout this repository [TRTC Web V4 SDK](https://github.com/TencentRTC/web)