CMake: Make all examples use Qt6:: instad of Qt:: in target_link_libraries

Qt module targets can be referenced both with Qt:: and Qt6::, e.g. Qt::Core vs Qt6::Core.

Qt:: was introduced for projects that need to stay compatible with Qt 5: See also Qt 5 and Qt 6 compatibility. But it has technical limitations, which affects e.g. Squish. So we shouldn't use it in Qt code ourselves, and especially not in Qt 6 examples.