All EF Core examples should use NRTs (nullable reference types) since that's now the default in .NET 6

Now that .NET 6 includes `enable` in the templates, it would be great if all the docs for EF Core could respect that as well.

Right now, if you follow e.g. the [Getting started](https://docs.microsoft.com/en-us/ef/core/get-started/overview/first-app?tabs=netcore-cli)-tutorial, the app will raise compiler warnings for the non-null model properties etc.

I'm sure this will be confusing for people new to .NET / EF Core.