fix(nextjs): disable the default-server x-powered-by option on the express server (#4180)

This way, when disabling the same option in the next.config.js it doesn't just revert to express

ISSUES CLOSED: #3961

Author: BigAB

packages/next/src/builders/server/lib/default-server.ts

@@ -16,6 +16,7 @@ export async function defaultServer(
   await app.prepare();
 
   const server: express.Express = express();
+  server.disable('x-powered-by');
 
   // Set up the proxy.
   if (proxyConfig) {