Update Flutter link to GitHub releases for consistency

- Change Flutter link from pub.dev to GitHub releases page
- Addresses PR feedback from rohit-710 about unifying platform links
- All platforms now consistently link to GitHub releases pages
- Update descriptive text to match other platform patterns

Co-Authored-By: ignacio.rivera@reown.com <ignacio.rivera@reown.com>

Author: devin-ai-integration[bot]

walletkit/upgrade/staying-up-to-date.mdx

@@ -30,5 +30,5 @@ Monitor JavaScript and React Native package updates:
 - [Reown WalletKit JS Releases](https://github.com/reown-com/reown-walletkit-js/releases)
 
 ### Flutter
-Track Flutter package versions on pub.dev:
-- [Reown WalletKit Flutter Package](https://pub.dev/packages/reown_walletkit)
+Check the latest Flutter releases for Flutter development:
+- [Reown Flutter Releases](https://github.com/reown-com/reown_flutter/releases)