Update required Visual Studio version in build docs

[xtqqczze]

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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• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
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  • I have considered the user experience from a tooling perspective and don't believe tooling will be impacted.
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[xtqqczze]

@TravisEz13 ready to merge

[ghost]

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[ghost]

🎉v7.1.0-preview.4 has been released which incorporates this pull request.:tada:

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