.NET 9: Code only EF with RP add inheritance and advanced topics

Update the code for [Razor Pages with Entity Framework Core in ASP.NET Core ](https://docs.microsoft.com/en-us/aspnet/core/data/ef-rp/intro) from .NET 6 to .NET 9

1. Update the .NET 6 code in the [cu90](https://github.com/dotnet/AspNetCore.Docs/tree/main/aspnetcore/data/ef-rp/intro/samples/cu90) to .NET 9
the `cu90` is a copy of the .NET 6 code

2. Quick review of code and update where necessary.
## Double bonus points
4. Add code for inheritance. Hopefully you can use some of the code from [Tutorial: Implement inheritance - ASP.NET MVC with EF Core](https://docs.microsoft.com/en-us/aspnet/core/data/ef-mvc/inheritance).
5. Consider adding code supporting [Tutorial: Learn about advanced scenarios - ASP.NET MVC with EF Core](https://docs.microsoft.com/en-us/aspnet/core/data/ef-mvc/advanced).
Once the preceding is complete, we'll port all the RP code and update the [Tutorial: Get started with EF Core in an ASP.NET MVC web app](https://docs.microsoft.com/en-us/aspnet/core/data/ef-mvc/intro)

Also tracked in https://github.com/dotnet/AspNetCore.Docs/issues/26505