MSVC: enable lvl3 warnings
Level 3 warnings include deprecations which are relevant for qt6 porting. Warnings for downcasts etc are also level3.
This solves a 10 year old fixme.
Apparently this was on by default until CMake 3.15. https://gitlab.kitware.com/cmake/cmake/-/issues/18317 https://gitlab.kitware.com/cmake/cmake/-/merge_requests/3250/diffs