Update 'working with the code' to remove reference to VS2017

I'm not sure you need to restate the version here, having got it specified in the list above, but if you do, then a vanilla 2017 won't build the source because of the c#8 stuff introduced recently.

.github/CONTRIBUTING.md

@@ -128,7 +128,7 @@ In order to build the Umbraco source code locally, first make sure you have the
   * npm v6.4.1+ (installed with Node.js)
   * [Git command line](https://git-scm.com/download/)
 
-The easiest way to get started is to open `src\umbraco.sln` in Visual Studio 2017 (version 15.9.7 or higher, [the community edition is free](https://www.visualstudio.com/thank-you-downloading-visual-studio/?sku=Community&rel=15) for you to use to contribute to Open Source projects). In Visual Studio, find the Task Runner Explorer (in the View menu under Other Windows) and run the build task under the gulpfile.
+The easiest way to get started is to open `src\umbraco.sln` in Visual Studio 2019 (version 16.3 or higher, [the community edition is free](https://www.visualstudio.com/thank-you-downloading-visual-studio/?sku=Community&rel=15) for you to use to contribute to Open Source projects). In Visual Studio, find the Task Runner Explorer (in the View menu under Other Windows) and run the build task under the gulpfile.
 
 Alternatively, you can run `build.ps1` from the Powershell command line, which will build both the backoffice (also known as "Belle") and the Umbraco core. You can then easily start debugging from Visual Studio, or if you need to debug Belle you can run `gulp dev` in `src\Umbraco.Web.UI.Client`. See [this page](BUILD.md) for more details.