Migrate to use QT6 (still supports QT5 5.15+)

[susnux]

This allows to use the current QT version 6 while keeping backwards compatibility with QT5 5.15 or later.

[DavidEGrayson]

Thanks. I'd like to switch over to Qt 6 and merge this in eventually, but I'll probably have to update the instructions in BUILDING.md and then test building on all the platforms we care about. You wrote "5.15" in the command to find Qt5, but we should probably just remove that, since we expect this to work with a wide variety of Qt5 versions.

[susnux]

I can adjust the building instructions as well :)

I think 5.15 was the version that introduced the versionless cmake macros? Took that comment from the qt docs.