Upgrade to Qt 6

[ltoenning]

This issue is a (maybe incomplete) collection of tasks that must be resolved to port swift to Qt 6:

• [x] Fix deprecation warnings
• [x] Replace call to QVariant::operator</QVariant::operator>. Qt6's QVariant does not support them but adds a new method QVariant::compare
  https://github.com/swift-project/pilotclient/blob/f209a1743c0e24463e803740febdc2a1a5337b49/src/blackmisc/variant.cpp#L170-L171
• [x] Remove or replace usage of bearer management (QNetworkConfigurationManager and QNetworkConfiguration )(https://doc.qt.io/qt-6/network-changes-qt6.html#bearer-management-is-removed)
• [x] Update usage of Qt Multimedia (for example QAudioDeviceInfo and QAudioDevice)
• [ ] Update deployment on Linux
• [ ] Update deployment on macOS
• [ ] Update deployment on Windows