Fix entity accessibility in EF Core Keys examples — use public instead of internal

[GPAPD]

The EF Core documentation for Keys
Currently shows example entity classes declared as internal.

Using internal limits the entity to the same assembly, which can be confusing, because entities are typically accessed from other parts of an application such as controllers, services, or unit tests.

This PR updates the examples to use public for entity classes, which is the recommended practice in real-world applications and better standard EF Core usage.

// Before
internal class Car
{
public string Id { get; set; }
public string Make { get; set; }
public string Model { get; set; }
}

// After
public class Car
{
public string Id { get; set; }
public string Make { get; set; }
public string Model { get; set; }
}

No functional changes are made.
This is a documentation improvement for clarity and correctness.

[GPAPD]

@dotnet-policy-service agree

[roji]

Thanks!