feat(remix-vercel): show deprecation warning on usage (#5964)

packages/remix-vercel/index.ts

@@ -1,4 +1,22 @@
 import "./globals";
 
+const alreadyWarned: Record<string, boolean> = {};
+const warnOnce = (message: string, key = message) => {
+  if (!alreadyWarned[key]) {
+    alreadyWarned[key] = true;
+    console.warn(message);
+  }
+};
+
+warnOnce(
+  "⚠️ REMIX FUTURE CHANGE: The `@remix-run/vercel` runtime adapter " +
+    "has been deprecated in favor of out of the box Vercel functionality and " +
+    "will be removed in Remix v2. Please update your code by removing " +
+    "`@remix-run/vercel` & `@vercel/node` from your `package.json`, removing " +
+    "your `server.js`/`server.ts` file, and removing the `server` & " +
+    "`serverBuildPath` options from your `remix.config.js`.",
+  "built-in-vercel-functionality"
+);
+
 export type { GetLoadContextFunction, RequestHandler } from "./server";
 export { createRequestHandler } from "./server";

templates/vercel/README.md

@@ -1,3 +1,9 @@
+> **Warning**  
+> The `@remix-run/vercel` runtime adapter has been deprecated in favor of out of
+> the box Vercel functionality and will be removed in Remix v2.  
+> This means you don't have to use the Vercel template & can just use the Remix
+> template instead.
+
 # Welcome to Remix!
 
 - [Remix Docs](https://remix.run/docs)