Upgrade build scripts to support VS2019. (mono/mono#17269)

Upgrade build scripts to support VS2019. (mono/mono#17269)

Changes will default Windows SDK version as well as Platform Toolset to
the default versions used in the targeted VS version. If the projects are
opened up in VS2015, it should default to Windows SDK 8.1 and v140, but if
the same projects are opened in VS2019, it will default to latest Windows SDK 10
and v142. This way the project files should adapt to used VS version, meaning
that we could still build them using VS2015 (what's currently used on CI) but also
using VS2017 and VS2019. There should not be a need to install any previous versions
of build tools, unless an older version is targeted. It is also possible to set
PlatformToolset when calling msbuild and that should adapt to corresponding default
Windows SDK version for targeted toolset version.

Commit makes many changes and adjustments, aligning all vcxproj files but changes
should not affect build output.

Commit migrated from https://github.com/mono/mono/commit/4f8a82c9fdebd7e9dcc5392a8b3a0fd129789bc0