Container issue for EF/MVC

This is the container issue for this doc. Reopen each issue you are working on so the owner is notified.

Before any of these issues are fixed, we'll do #26505, which will resolve many of the following issues.

* This: asp.net core 3.1 fails to create migration - The entity type 'Instructor' cannot be mapped to a table because it is derived from 'Person'. Only base entity types can be mapped to a table.
* Manual migration code is wrong for resulting model #21860
* Adding Person breaks unless you also comment out or delete sections of the schoolcontext.cs #20671
* Getting type cannot be instantiated error for abstract base class #21430
* Home Controller About Custom Code is wrong #21861
* Using .net5, the FromSQL produces an error #21783
* SingleOrDefaultAsync throws if multiple rows returned, not null #20754
* Examine SQL queries... not working #21862
* ASP.NET MVC with EF Core Tutorial: Implement inheritance #22047
* Incomplete code changes. #22828

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#### Відомості про документ

⚠ *Не змінюйте цей розділ. Він необхідний для зв’язування питань на платформах docs.microsoft.com ➟ GitHub.*

* ID: 55345ff6-3c81-969e-3c92-29c24d78d2ca
* Version Independent ID: aa826512-74a5-752a-0ad0-0f8133f6c39e
* Content: [Tutorial: Implement inheritance - ASP.NET MVC with EF Core](https://docs.microsoft.com/uk-ua/aspnet/core/data/ef-mvc/inheritance?view=aspnetcore-3.1#feedback)
* Content Source: [aspnetcore/data/ef-mvc/inheritance.md](https://github.com/dotnet/AspNetCore.Docs/blob/master/aspnetcore/data/ef-mvc/inheritance.md)
* Product: **aspnet-core**
* Technology: **aspnetcore-data**
* GitHub Login: @Rick-Anderson
* Microsoft Alias: **riande**