Add unsupported OS warning UX when tor-browser upgrade cannot happen due to user's device no longer meeting min-spec
We have logic in UpdateService.jsm that detects when the user's OS version is incompatible with a pending update, but we don't have any UX to tell the user about it.
As part of this we should also add a debug pref for exercising this code path (similar to how we use torbrowser.debug.censorship_level
in TorConnect.jsm)