Bug#1121645: freecad: Build against qt6

[Gregor Riepl]

> would it be possible to build against qt6?
The upstream Flatpack is built on Qt6, so there doesn't seem to be any hard requirement to use Qt5: https://github.com/flathub/org.freecad.FreeCAD/blob/master/org.freecad.FreeCAD.yaml#L51

There's also a tag on the upstream bug tracker for Qt6 issues: https://github.com/FreeCAD/FreeCAD/issues?q=is%3Aissue%20state%3Aopen%20label%3A%223rd%20party%3A%20Qt%206%22
While some issue may indicate that there are still some bugs, it doesn't look like using Qt6 is generally unadvisable.

A Qt6 build would be appreciated!