MSVC.README.rst: Update Visual Studio build information

Hi,

This attempts to update MSVC.README.rst on the following parts:

• Drop obsolete info. Officially, we require Visual Studio 2015 or later to build and run G-I.
• Recommend that the latest Visual Studio 2019 (or later) is used when building introspection for C++ items, for the best build experience. Bizarre errors popped up when building HarfBuzz's introspection files when using Visual Studio 2015 and 2017 unless one is building from a release tarball running Meson and the g-ir-scanner using Python-3.7.x, as later Visual Studio 2019 releases and later had a reworked preprocessor (note that it's perfectly fine to build G-I using Visual Studio 2015 and 2017, and for any Visual Studio 2015+ with the same bitness to use/run such builds).

It would be best if someone can take a look at !403, since Python-3.12.x is around the corner.

This will close #466 (closed), since it will likely become a won't-fix unless someone knows a better solution.

With blessings, thank you!