Change the build system to enable building with Qt6
This was built with:
-DQT_MAJOR_VERSION=6
-DEXCLUDE_DEPRECATED_BEFORE_AND_AT=5.90.0
-DKF_DISABLE_DEPRECATED_BEFORE_AND_AT=0x055a00
Can't use QNetworkConfigurationManager with Qt6 in ProxyScout as there is currently no replacement for the functionality needed there.