Update 3.1-to-5.0 migration guide for Preview 6 (#18965)

Author: scottaddie

aspnetcore/includes/net-core-prereqs-mac-5.0.md

@@ -1 +1,2 @@
-ASP.NET Core 5.0 isn't yet supported in Visual Studio for Mac.
+* [Visual Studio for Mac Preview](https://visualstudio.microsoft.com/vs/mac/)
+* [!INCLUDE [.NET 5.0 SDK](~/includes/5.0-SDK.md)]

aspnetcore/includes/net-core-prereqs-vs-5.0.md

@@ -1,2 +1,2 @@
-* [Visual Studio 2019 16.6 or later](https://visualstudio.microsoft.com/downloads/?utm_medium=microsoft&utm_source=docs.microsoft.com&utm_campaign=inline+link&utm_content=download+vs2019) with the **ASP.NET and web development** workload
+* [Visual Studio 2019 16.7 or later](https://visualstudio.microsoft.com/downloads/?utm_medium=microsoft&utm_source=docs.microsoft.com&utm_campaign=inline+link&utm_content=download+vs2019) with the **ASP.NET and web development** workload
 * [!INCLUDE [.NET 5.0 SDK](~/includes/5.0-SDK.md)]

aspnetcore/migration/31-to-50.md

@@ -4,7 +4,7 @@ author: scottaddie
 description: Learn how to migrate an ASP.NET Core 3.1 project to ASP.NET Core 5.0.
 ms.author: scaddie
 ms.custom: mvc
-ms.date: 06/11/2020
+ms.date: 06/25/2020
 no-loc: [Blazor, "Identity", "Let's Encrypt", Razor, SignalR]
 uid: migration/31-to-50
 ---
@@ -41,7 +41,7 @@ If you rely upon a [global.json](/dotnet/core/tools/global-json) file to target
 {
   "sdk": {
 -    "version": "3.1.200"
-+    "version": "5.0.100-preview.5.20279.10"
++    "version": "5.0.100-preview.6.20318.15"
   }
 }
 ```
@@ -69,8 +69,8 @@ In the project file, update each `Microsoft.AspNetCore.*` and `Microsoft.Extensi
 <ItemGroup>
 -    <PackageReference Include="Microsoft.AspNetCore.Mvc.NewtonsoftJson" Version="3.1.2" />
 -    <PackageReference Include="Microsoft.Extensions.Caching.Abstractions" Version="3.1.2" />
-+    <PackageReference Include="Microsoft.AspNetCore.Mvc.NewtonsoftJson" Version="5.0.0-preview.5.20279.2" />
-+    <PackageReference Include="Microsoft.Extensions.Caching.Abstractions" Version="5.0.0-preview.5.20278.1" />
++    <PackageReference Include="Microsoft.AspNetCore.Mvc.NewtonsoftJson" Version="5.0.0-preview.6.20312.15" />
++    <PackageReference Include="Microsoft.Extensions.Caching.Abstractions" Version="5.0.0-preview.6.20305.6" />
 </ItemGroup>
 ```