calls-notifier: Use official notification category

See https://gitlab.freedesktop.org/xdg/xdg-specs/-/merge_requests/50

Part-of: <https://gitlab.gnome.org/GNOME/calls/-/merge_requests/748>

src/calls-notifier.c

@@ -59,7 +59,7 @@ notify (CallsNotifier *self, CuiCall *call)
   gboolean got_id;
   gboolean got_contact;
 
-  g_notification_set_category (notification, "x-gnome.call.unanswered");
+  g_notification_set_category (notification, "call.unanswered");
   /* TODO: We need to update the notification when the contact name changes
      We would need to resend the notification in this case, as changing the properties
      after having called g_application_send_notification() will have no effect.