add --upgrade flag and print message showing up to upgrade

[jsjoeio]

We discussed adding the --upgrade flag to upgrade. A feature like this used to exist in code-server but presenting a lot of issues.

To make a slight improvement, I propose we add the flag which prints out a message explaining how to upgrade. That would be a slight DX improvement to the current approach while limiting an increase in maintenance

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no activity occurs in the next 5 days.