Add v2 maintenance mode warning (#10630)

Author: brophdawg11

README.md

@@ -1,5 +1,12 @@
 # Welcome to Remix!
 
+> [!IMPORTANT]
+> With the release of [React Router v7](https://remix.run/blog/react-router-v7) we are sun-setting continued development/maintenance on Remix v2. If you have not already [upgraded to React Router v7](https://reactrouter.com/upgrading/remix), we recommend you do so. We've tried to make the upgrade process as smooth as possible with our [Future Flags](https://remix.run/docs/en/main/start/future-flags).
+>
+> We plan to continue to address bugs that pose security concerns or prevent upgrading to React Router v7. If you have a bug in one of those categories, please open an issue in this repository and prefix the title with `(v2):`.
+>
+> For all other issues, ongoing maintenance will be happening in React Router v7, so please o[pen issues (bugs) and/or discussions (feature requests) [in that repo](https://github.com/remix-run/react-router/issues/new?template=bug_report.yml).
+
 We are happy you're here!
 
 [Remix](https://remix.run) is a full stack web framework that lets you focus on the user interface and work back through web fundamentals to deliver a fast, slick, and resilient user experience that deploys to any Node.js server and even non-Node.js environments at the edge like Cloudflare Workers.