Add notes about behavior changes to model binding in 5.0 (#19971)

* Add notes about behavior changes to model binding in 5.0

* Update 31-to-50.md

* Apply suggestions from code review

Co-authored-by: Scott Addie <10702007+scottaddie@users.noreply.github.com>

Co-authored-by: Rick Anderson <3605364+Rick-Anderson@users.noreply.github.com>
Co-authored-by: Scott Addie <10702007+scottaddie@users.noreply.github.com>

Author: 3 people

aspnetcore/migration/31-to-50.md

@@ -120,3 +120,24 @@ As part of the move to ".NET" as the product name, the Docker images moved from
 ## Review breaking changes
 
 For breaking changes from .NET Core 3.1 to .NET 5.0, see [Breaking changes for migration from version 3.1 to 5.0](/dotnet/core/compatibility/3.1-5.0). ASP.NET Core and Entity Framework Core are also included in the list.
+
+## Changes to model binding in ASP.NET Core MVC and Razor Pages
+
+### DateTime values are model bound as UTC times
+
+In ASP.NET Core 3.1 and earlier, `DateTime` values were model bound as local time where the timezone was determined by the server. `DateTime` values bound from input formatting (JSON) and `DateTimeOffset` values were bound as UTC timezones. In ASP.NET Core 5.0 and later, model binding consistently binds `DateTime` values with the UTC timezone.
+
+To retain the previous behavior, remove the `DateTimeModelBinderProvider` as part of the application startup:
+
+```csharp
+services.AddControllersWithViews(options => options.ModelBinderProviders.RemoveType<DateTimeModelBinderProvider>());
+```
+
+### ComplexObjectModelBinderProvider \ ComplexObjectModelBinder replace ComplexTypeModelBinderProvider \ ComplexTypeModelBinder 
+
+As part of adding support for model binding record types, the `ComplexTypeModelBinderProvider` is annotated as obsolete and is no longer registered by default. Applications that previously relied on the presence of the `ComplexTypeModelBinderProvider` in the `ModelBinderProviders` collection need to be updated to reference the new binder provider:
+
+```diff
+- var complexModelBinderProvider = options.ModelBinderProviders.OfType<ComplexTypeModelBinderProvider>();
++ var complexModelBinderProvider = options.ModelBinderProviders.OfType<ComplexObjectModelBinderProvider>();
+```