[nemo-connectivity] Use shared instance of NetworkManager. JB#61563

[pvuorela]

Depends on:
sailfishos/libconnman-qt#27

[LaakkonenJussi]

LGTM

[LaakkonenJussi]

That deprecation change seems to be a success in sailfishos/libconnman-qt#27 - this popped in the build.

connectionhelper.cpp:83:39: warning: ‘static NetworkManager* NetworkManagerFactory::createInstance()’ is deprecated: Use NetworkManager::sharedInstance() [-Wdeprecated-declarations]
   83 |     , m_netman(NetworkManagerFactory::createInstance())
      |                                       ^~~~~~~~~~~~~~
In file included from connectionhelper.cpp:48:
/usr/include/connman-qt5/networkmanager.h:29:88: note: declared here
   29 |     Q_DECL_DEPRECATED_X("Use NetworkManager::sharedInstance()") static NetworkManager* createInstance();
      |                                                                                        ^~~~~~~~~~~~~~