ci: Adapt warning configuration to new release of MSVC 19

ci: Adapt warning configuration to new release of MSVC 19

Recently, MSVC introduced a new warning C5240 that apparently refers
to standard-compliant ordering of attributes and decl-specifies.
Unfortunately, this warning is triggered by Qt headers, in particular:

C:\CraftMaster\windows-msvc2019_64-cl\include\qt5\QtGui/qcolor.h(233):
    warning C5240: 'nodiscard': attribute is ignored in this syntactic position

Since we cannot fix this ourselves, make the warning non-fatal for now.
This fixes our CI failing with the new MSVC version.

Reenable C4244, which is no longer triggered by Quassel code, and add
some reasoning for the remaining warnings.