[Development] Let's drop MSVC 2019 for dev (6.8)

[Kai Köhne]

> MSVC2019 and MSVC2022 is supposed to be compatible. So in theory at least
> packages made with MSVC2022 is still usable for MSVC2019 as well. It is just a
> question of which compiler we use to generate the binaries with.

That's not the case, at least according to Microsoft (https://learn.microsoft.com/en-us/cpp/porting/binary-compat-2015-2017?view=msvc-170):
     
     You can mix binaries built by different versions of the v140, v141, v142, and v143 toolsets. However, you must link by using a toolset at least as recent as the most recent binary in your app. 

As almost all Qt users link against the Qt libraries, they need to, therefore, use at least the Microsoft Compiler version that the CI uses.

Kai