Mark PsPdfKitAnnotation as obsolete

Added an [Obsolete] attribute to the PsPdfKitAnnotation property
in the DocSignedNotification class, indicating that the PSPDFKit
library is deprecated.

Reorganized using directives in DocumentReceiverElementService.cs
to improve consistency and maintain code readability.

EnvelopeGenerator.Application/Common/Notifications/DocSigned/DocSignedNotification.cs

@@ -20,6 +20,7 @@ public record DocSignedNotification : INotification, ISendMailNotification
     /// <summary>
     /// The PSPDFKit annotation data.
     /// </summary>
+    [Obsolete("The PSPDFKit library is deprecated.")]
     public PsPdfKitAnnotation? PsPdfKitAnnotation { get; init; }
 
     /// <summary>