gio: Stop using deprecated NM PropertiesChanged signal
Use the org.freedesktop.DBus.Properties interface to listen to PropertiesChanged signals on /org/freedesktop/NetworkManager.
NetworkManager used to provide its own legacy PropertiesChanged signal, but that was dropped in https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/853
We could probably do some additional checking or fallback for NM versions < 1.3.1 (from 2010), but over 10 years seems enough time.
Fixes: #2505 (closed) Bug-Ubuntu: https://bugs.launchpad.net/bugs/1946196