Remove StatusChangedWhen from DocumentStatus entity

The StatusChangedWhen property, previously required and mapped
to the "STATUS_CHANGED_WHEN" column, has been removed from
the DocumentStatus entity in the EnvelopeGenerator.Domain.Entities
namespace. This change simplifies the entity and its mapping.

EnvelopeGenerator.Domain/Entities/DocumentStatus.cs

@@ -35,7 +35,6 @@ namespace EnvelopeGenerator.Domain.Entities
         [Column("STATUS")]
         public Constants.DocumentStatus Status { get; set; }
 
-        [Required]
         [Column("STATUS_CHANGED_WHEN", TypeName = "datetime")]
         public DateTime? StatusChangedWhen { get; set; }