Update GUI to Qt6

Since Qt 6 has been released for a year now, it is time to move to it for longer term support and better maintenance. Qt 6 recommends using CMake for the build rather then qmake.

Task:

• Port qmake .pro to CMakeList.txt
• Test building on ubuntu with Qt6 with the current qmake project
• Test building on MacOS with Qt6 with current qmake project
• Test cross building for windows using mxe (static)
• update the build container image
• fix gitlab ci builds