gio: Correct the "available in" for GNetworkMonitor

They were marked as available in all versions when the main interface
was actually added in glib 2.32.

https://bugzilla.gnome.org/show_bug.cgi?id=664562
gio: add network connectivity state to GNetworkMonitor

Add a property to GNetworkMonitor indicating the level of network
connectivity: none/local, limited, stuck behind a portal, or full.

The default implementation just returns none or full depending on the
value of is-available.

https://bugzilla.gnome.org/show_bug.cgi?id=664562
gio: Add GNetworkMonitor impl based on NetworkManager

Which implements the new GNetworkConnectivity property.

https://bugzilla.gnome.org/show_bug.cgi?id=664562