Fix logged aiohttp warning about "app key"

I saw this warning but havn't investigated what its about or how to fix - it would make sense to do though!

```
  /opt/python/lib/python3.11/site-packages/dask_gateway_server/app.py:179: NotAppKeyWarning: It is recommended to use web.AppKey instances for keys.
  https://docs.aiohttp.org/en/stable/web_advanced.html#application-s-config
    self.app["gateway"] = self
```