[FEATURE] Immediately update server when nobody is connected

I run a Valheim server with LGSM. Using crontab I schedule the 'vhserver update' command every 24 hours at 0400. If an update is found, it will force update the server at that time.

This is far from ideal, as updates can take up to 24 hours to be applied this way.

- I would very much like a feature where the server is immediately updated after an update is found, but only if the server is empty.
- It would also be useful to be able to message connected users that an update has been found and the server needs to be empty to apply the update immediately.