Port away from QNetworkConfigurationManager

QNetworkConfigurationManager was removed from Qt6 and it's better to
check the status of the sync job anyway. Only two issues:

* The timeout in quotient is quite high so it might take up to one
  minute before the message appear.
* Only sync job are listened but since they are continuously done, this
  is not a big issue and other job are affected by the same issue of an
  high timeout anyway.

Fix #414