Update required Visual Studio version in build docs (#12628)
Update required Visual Studio version in build docs (#12628)

# PR Summary

* Update build docs to specify Visual Studio 2019 16.6 Preview 2 as a requirement, due to the of `net5.0` TFM.
* Remove dependancy on "Common Tools for Visual C++" as there is no longer C++ code in the repository.

## PR Context

Visual Studio 2019 >= 16.6 Preview 2 is required since #12486.
see also: #12514

## PR Checklist

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JamesWTruher/PowerShell-1,daxian-dbw/PowerShell,daxian-dbw/PowerShell,PaulHigin/PowerShell,JamesWTruher/PowerShell-1,TravisEz13/PowerShell,TravisEz13/PowerShell,daxian-dbw/PowerShell,JamesWTruher/PowerShell-1,PaulHigin/PowerShell,PaulHigin/PowerShell,TravisEz13/PowerShell,PaulHigin/PowerShell,JamesWTruher/PowerShell-1,TravisEz13/PowerShell,daxian-dbw/PowerShell