[CoreData] Move CoreData into the .NET age. (#13718)

* Remove obsolete API from .NET.
* Change API marked with XAMCORE_4_0 due to naming problems to be in .NET.
* Change API marked with XAMCORE_4_0 due to using non-generic NSDictionary to be
  in XAMCORE_5_0 instead (yay!). This is because of #13704, which can make using
  generic NSDictionary in API buggy, and I feel it's a bit too risky to change this
  for .NET with the time we have available (no time to fix #13704). Additionally,
  moving this to XAMCORE_5_0 makes it possible to keep grepping for XAMCORE_4_0 to
  see what's left. Update all the CoreData API to be better as defined by our XAMCORE_4_0
  define. Mostly using generic NSSet/NSDictionary types instead of the non-generic
  ones, and other misc naming improvements.
* Change API marked with XAMCORE_4_0 due to both of the above to do both of the
  above - add a version of the naming issue fixed for .NET + a version with the generic
  dictionary for XAMCORE_5_0.

src/CoreData/Obsolete.cs

@@ -30,7 +30,7 @@ public NSPersistentStore ()
 		}
 	}
 #endif
-#if !XAMCORE_4_0
+#if !NET
 	public partial class NSCoreDataCoreSpotlightDelegate {
 
 		[Obsolete ("Default constructor is not available")]