Remove all custom server integration examples (vercel#42549)

Fixes @leerob's comment vercel#42110 (comment)

~~Deprecated~~ Removed all custom server integration examples (Express, Fastify, etc.) in favour of the main `custom-server` example, ~~and left behind a helpful `README.md` in case they might be linked to.~~

## Documentation / Examples

- [X] Make sure the linting passes by running `pnpm build && pnpm lint`
- [X] The "examples guidelines" are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

Author: maxproske

docs/advanced-features/custom-server.md

@@ -7,9 +7,7 @@ description: Start a Next.js app programmatically using a custom server.
 <details>
   <summary><b>Examples</b></summary>
   <ul>
-    <li><a href="https://github.com/vercel/next.js/tree/canary/examples/custom-server-express">Express integration</a></li>
-    <li><a href="https://github.com/vercel/next.js/tree/canary/examples/custom-server-hapi">Hapi integration</a></li>
-    <li><a href="https://github.com/vercel/next.js/tree/canary/examples/custom-server-koa">Koa integration</a></li>
+    <li><a href="https://github.com/vercel/next.js/tree/canary/examples/custom-server">Custom Server</a></li>
     <li><a href="https://github.com/vercel/next.js/tree/canary/examples/ssr-caching">SSR Caching</a></li>
   </ul>
 </details>