Update nginx recommendations (#26924)

- `Connection` and `Upgrade` [needed for
websockets](https://www.nginx.com/blog/websocket-nginx/)
- ~~`X-Real-IP` unnecessary and duplicate with `X-Forwarded-For`. [chi
checks both
headers](https://github.com/go-chi/chi/blob/master/middleware/realip.go),
but XFF is definitely the more "standard" one.~~

docs/content/administration/reverse-proxies.en-us.md

@@ -29,6 +29,8 @@ server {
     location / {
         client_max_body_size 512M;
         proxy_pass http://localhost:3000;
+        proxy_set_header Connection $http_connection;
+        proxy_set_header Upgrade $http_upgrade;
         proxy_set_header Host $host;
         proxy_set_header X-Real-IP $remote_addr;
         proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;