Update Enumeration and CardType class

[mfcallahan]

Per suggestions in dotnet/docs repository issue #18496, update the Microsoft.eShopOnContainers.Services.Ordering.Domain.SeedWork.Enumeration class the and Microsoft.eShopOnContainers.Services.Ordering.Domain.AggregatesModel.BuyerAggregate.CardType class to use modern C# features:

• nameof expression
• pattern matching
• use expression bodied members consistently

A corresponding pull request for the documentation is also pending.