If a device has multiple connections set up, then at most one of
those can be active at a time, which is why they are presented
as radio items.
In contrast, VPN connections are not mutually exclusive, each can
be turned on or off independently. Setting :radio-mode on them
currently means that VPN connections can be activated, but never
disabled.
So instead of abusing the :radio-mode property to give VPN items
the UI we want, use regular items that reflect the desired behavior
and explicitly set up the UI the way we want.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2426>
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