Add a migration scenario for Blazor (#34011)

Author: guardrex

aspnetcore/migration/80-to-90/includes/blazor.md

@@ -1 +1,37 @@
-Blazor migration guidance will appear here prior to the release of .NET 9, which is scheduled for November, 2024.
+### Adopt simplified authentication state serialization for Blazor Web Apps
+
+Blazor Web Apps can optionally adopt [simplified authentication state serialization](xref:aspnetcore-9#simplified-authentication-state-serialization-for-blazor-web-apps).
+
+In the server project:
+
+* Remove the Persisting Authentication State Provider (`PersistingAuthenticationStateProvider.cs`).
+
+* Remove the service registration from the `Program` file. Instead, chain a call to <xref:Microsoft.Extensions.DependencyInjection.WebAssemblyRazorComponentsBuilderExtensions.AddAuthenticationStateSerialization%2A> on <xref:Microsoft.Extensions.DependencyInjection.RazorComponentsServiceCollectionExtensions.AddRazorComponents%2A>:
+
+  ```diff
+  - builder.Services.AddScoped<AuthenticationStateProvider, PersistingAuthenticationStateProvider>();
+
+    builder.Services.AddRazorComponents()
+        .AddInteractiveServerComponents()
+        .AddInteractiveWebAssemblyComponents()
+  +     .AddAuthenticationStateSerialization();
+  ```
+
+The API only serializes the server-side name and role claims for access in the browser. To include all claims, set <xref:Microsoft.AspNetCore.Components.WebAssembly.Server.AuthenticationStateSerializationOptions.SerializeAllClaims> to `true`:
+
+```csharp
+.AddAuthenticationStateSerialization(options => options.SerializeAllClaims = true);
+```
+
+In the client project (`.Client`):
+
+* Remove the Persistent Authentication State Provider (`PersistentAuthenticationStateProvider.cs`).
+
+* Remove the service registration from the `Program` file. Instead, call <xref:Microsoft.Extensions.DependencyInjection.WebAssemblyAuthenticationServiceCollectionExtensions.AddAuthenticationStateDeserialization%2A> on the service collection:
+
+  ```diff
+  - builder.Services.AddSingleton<AuthenticationStateProvider, PersistentAuthenticationStateProvider>();
+  + builder.Services.AddAuthenticationStateDeserialization();
+  ```
+
+For more information, see <xref:aspnetcore-9#simplified-authentication-state-serialization-for-blazor-web-apps>.