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