Remove Qt5 support (also from README)

Luigi Toscanorequested to merge
work/ltoscano/remove-qt5 into master

Set the minimum Qt to 6, remove the usage of QT_MAJOR_VERSION, remove the Qt version checking which references very old releases, and adapt the CI (including enabling it for windows/Qt6).

README: make the hardcoded reference to the Frameworks version more generic and move away from the anongit reference.

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