Add RCL migration to 5.0 migration topic (#20506)

* Update 31-to-50.md

* Updates

* Updates

* Updates

* Updates

Co-authored-by: Luke Latham <llatham@aquent.com>

aspnetcore/migration/31-to-50.md

@@ -199,6 +199,38 @@ To resolve the error:
 1. In the Azure portal, access the [app's manifest](/azure/active-directory/develop/reference-app-manifest).
 1. Set the [`allowPublicClient`](/azure/active-directory/develop/reference-app-manifest#allowpublicclient-attribute) attribute to `null` or `true`.
 
+## Update Razor class libraries (RCLs)
+
+Migrate Razor class libraries (RCLs) to take advantage of new APIs or features that are introduced as part of ASP.NET Core 5.0. 
+
+To update a RCL that targets components:
+
+1. Update the following properties in the project file:
+
+   ```diff
+   <Project Sdk="Microsoft.NET.Sdk.Razor">
+    
+     <PropertyGroup>
+   -     <TargetFramework>netstandard2.0</TargetFramework>
+   -     <RazorLangVersion>3.0</RazorLangVersion>
+   +     <TargetFramework>net5.0</TargetFramework>
+     </PropertyGroup>
+   ```
+
+1. Update other packages to their latest versions. The latest versions can be found at [NuGet.org](https://www.nuget.org).
+
+To update an RCL targeting MVC, update the following properties in the project file:
+
+```diff
+<Project Sdk="Microsoft.NET.Sdk.Razor">
+
+  <PropertyGroup>
+-    <TargetFramework>netcoreapp3.1</TargetFramework>
++    <TargetFramework>net5.0</TargetFramework>
+    <AddRazorSupportForMvc>true</AddRazorSupportForMvc>
+  </PropertyGroup>
+```
+
 ## Update package references
 
 In the project file, update each [Microsoft.AspNetCore.*](https://www.nuget.org/packages?q=Microsoft.AspNetCore.*), [Microsoft.Extensions.*](https://www.nuget.org/packages?q=Microsoft.Extensions.*), and [System.Net.Http.Json](https://www.nuget.org/packages/System.Net.Http.Json) package reference's `Version` attribute to 5.0.0 or later. For example: