Analyzer warning for using required on DbSet properties

In the following:

```c#
public class MyContext : DbContext
{
public required DbSet Blogs { get; set; }
}
```

It's common for users to use `required`, since the property is non-nullable and uninitialized: in normal circumstances, such a property triggers a warning for an uninitialized NRT property. However, EF comes with a diagnostics suppressor that removes the NRT warning, and the `required` just gets in the way: you can't simply initialize MyContext (you must also initialize Blogs), although the DbContext constructor takes care of injecting the property.

In other words, we have a pit of failure here... We should issue a warning for this case, with a code fix to remove the `required`.

/cc @DamianEdwards @davidfowl @AndriySvyryd, as discussed offline.