Update custom-routes-proxying to use fallback rewrites (#23610)

This updates to use the new `fallback` rewrites support instead of the previous no-op rewrite workaround. 


## Documentation / Examples

- [x] Make sure the linting passes

examples/custom-routes-proxying/next.config.js

@@ -1,16 +1,12 @@
 module.exports = {
   async rewrites() {
-    return [
-      // we need to define a no-op rewrite to trigger checking
-      // all pages/static files before we attempt proxying
-      {
-        source: '/:path*',
-        destination: '/:path*',
-      },
-      {
-        source: '/:path*',
-        destination: `https://custom-routes-proxying-endpoint.vercel.app/:path*`,
-      },
-    ]
+    return {
+      fallback: [
+        {
+          source: '/:path*',
+          destination: `https://custom-routes-proxying-endpoint.vercel.app/:path*`,
+        },
+      ],
+    }
   },
 }