[th/dbus-drop-legacy-properties-changed] core: drop deprecated PropertiesChanged D-Bus signal (API break)

Thomas Haller requested to merge th/dbus-drop-legacy-properties-changed into main

D-Bus 1.3.1 (2010) introduced the standard "PropertiesChanged" signal on "org.freedesktop.DBus.Properties". NetworkManager is old, and predates this API. From that time, it still had it's own PropertiesChanged signal that are emitted together with the standard ones.

These own signals are deprecated for a long time already ([1], 2016), and are hopefully not used by anybody anymore. libnm-glib was using them and relied on them, but that library is gone. libnm does not use them and neither does plasma-nm.

Hopefully no users are left that are affected by this API break.

[1] https://gitlab.freedesktop.org/NetworkManager/NetworkManager/commits/6fb917178aa19c61e909957f5146aa4565e0cb2f

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