[AC-2678] Enterprise to Families Sponsorship Bugs (#4118)

* Removed prorationDate as it wasn't used, and wasn't needed

* Fixed logic to detect if a subscription was sponsored

* Moved OrganizationSponsorshipsController.cs to Billing folder

src/Core/Models/Business/SecretsManagerSubscriptionUpdate.cs

@@ -29,11 +29,6 @@ public class SecretsManagerSubscriptionUpdate
     /// </summary>
     public int? MaxAutoscaleSmServiceAccounts { get; set; }
 
-    /// <summary>
-    /// The proration date for the subscription update (optional)
-    /// </summary>
-    public DateTime? ProrationDate { get; set; }
-
     /// <summary>
     /// Whether the subscription update is a result of autoscaling
     /// </summary>