Qt6x: Make Qt-6.5 minimum requirement
Make Qt-6.5 minimum requirement in qtver=qt6x builds.
Many of our testing distributions have now updated Qt to 6.6 (from 6.4), so it would be good time to bump the minimum requirement, and to go directly to 6.6.
- Rebased on top of https://redmine.freeciv.org/issues/721
Make Qt-6.5 minimum requirement in qtver=qt6x builds.