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

- [x] [PR has a meaningful title](https://github.com/PowerShell/PowerShell/blob/master/.github/CONTRIBUTING.md#pull-request---submission)
    - Use the present tense and imperative mood when describing your changes
- [x] [Summarized changes](https://github.com/PowerShell/PowerShell/blob/master/.github/CONTRIBUTING.md#pull-request---submission)
- [x] [Make sure all `.h`, `.cpp`, `.cs`, `.ps1` and `.psm1` files have the correct copyright header](https://github.com/PowerShell/PowerShell/blob/master/.github/CONTRIBUTING.md#pull-request---submission)
- [x] This PR is ready to merge and is not [Work in Progress](https://github.com/PowerShell/PowerShell/blob/master/.github/CONTRIBUTING.md#pull-request---work-in-progress).
    - If the PR is work in progress, please add the prefix `WIP:` or `[ WIP ]` to the beginning of the title (the `WIP` bot will keep its status check at `Pending` while the prefix is present) and remove the prefix when the PR is ready.
- **[Breaking changes](https://github.com/PowerShell/PowerShell/blob/master/.github/CONTRIBUTING.md#making-breaking-changes)**
    - [x] None
    - **OR**
    - [ ] [Experimental feature(s) needed](https://github.com/MicrosoftDocs/PowerShell-Docs/blob/staging/reference/6/Microsoft.PowerShell.Core/About/about_Experimental_Features.md)
        - [ ] Experimental feature name(s): <!-- Experimental feature name(s) here -->
- **User-facing changes**
    - [x] Not Applicable
    - **OR**
    - [ ] [Documentation needed](https://github.com/PowerShell/PowerShell/blob/master/.github/CONTRIBUTING.md#pull-request---submission)
        - [ ] Issue filed: <!-- Number/link of that issue here -->
- **Testing - New and feature**
    - [x] N/A or can only be tested interactively
    - **OR**
    - [ ] [Make sure you've added a new test if existing tests do not effectively test the code changed](https://github.com/PowerShell/PowerShell/blob/master/.github/CONTRIBUTING.md#before-submitting)
- **Tooling**
    - [x] I have considered the user experience from a tooling perspective and don't believe tooling will be impacted.
    - **OR**
    - [ ] I have considered the user experience from a tooling perspective and enumerated concerns in the summary. This may include:
        - Impact on [PowerShell Editor Services](https://github.com/PowerShell/PowerShellEditorServices) which is used in the [PowerShell extension](https://github.com/PowerShell/vscode-powershell) for VSCode (which runs in a different PS Host).
        - Impact on Completions (both in the console and in editors) - one of PowerShell's most powerful features.
        - Impact on [PSScriptAnalyzer](https://github.com/PowerShell/PSScriptAnalyzer) (which provides linting & formatting in the editor extensions).
        - Impact on [EditorSyntax](https://github.com/PowerShell/EditorSyntax) (which provides syntax highlighting with in VSCode, GitHub, and many other editors).

docs/building/windows-core.md

@@ -18,8 +18,7 @@ This guide assumes that you have recursively cloned the PowerShell repository an
 
 ### Visual Studio
 
-You will need to install an edition of Visual Studio 2015 (Community, Enterprise, or Professional) with the optional feature 'Common Tools for Visual C++' installed.
-The free Community edition of Visual Studio 2015 can be downloaded [here](https://www.visualstudio.com/visual-studio-community-vs/).
+This repository requires at least Visual Studio 2019 16.6 Preview 2. The free Community edition of Visual Studio can be downloaded from [Microsoft](https://visualstudio.microsoft.com/downloads/).
 
 ### Visual Studio Code