Update eShop code to show EF best practices

https://learn.microsoft.com/en-us/dotnet/architecture/cloud-native/introduce-eshoponcontainers-reference-app

Traditionally, this code shows implementations of some patterns that do not promote good use of EF Core. This code is also used as a reference for DDD concepts. Going forward, we should:

- [ ] Finish updating eShop for best practices
- [ ] Consider what DDD guidance we should give and where
- [ ] Redirect people away from old guidance