COMP: Fix Qt >= 6 updateSetting signal connection

[codeling]

On starting a slicer built against Qt6 with the current CTK main branch, it reports many signal connection failures like this:

QObject::connect: No such signal QSignalMapper::mapped(QString) in SLICERBUILDDIR\CTK\Libs\Widgets\ctkSettingsPanel.cpp:328
QObject::connect:  (receiver name: 'qSlicerSettingsGeneralPanel')

This is because the mapped() signal was removed with Qt6.

This pull request makes CTK use the successor mappedString instead (and switches to the new style of connecting signals for it, which would cause a compliation failure if the connection cannot be established).