Migrate from iText to GdPicture for PDF processing

Replaced iText with GdPicture for PDF manipulation, including:
- Added `Width` and `Height` properties to `IDocReceiverElement`.
- Updated `Pdf.cs` to use GdPicture's API for drawing and exporting.
- Introduced GdPicture licensing management with `EnsureInitialized`.
- Refactored `Background` method to align with GdPicture's coordinate system.
- Simplified resource management in `Dispose` and `DisposeAsync`.

Updated project files:
- Modified target frameworks in `EnvelopeGenerator.PdfEditor.csproj` to remove `net7.0` and `net9.0`, and added `net8.0-windows`.
- Replaced iText and BouncyCastle dependencies with GdPicture packages.
- Added `Microsoft.Bcl.AsyncInterfaces` to `EnvelopeGenerator.Web.csproj`.

Performed general cleanup and refactoring to remove unused iText code.

EnvelopeGenerator.Domain/Interfaces/IDocReceiverElement.cs

@@ -7,5 +7,9 @@
         double X { get; set; }
 
         double Y { get; set; }
+
+        double Width { get; set; }
+
+        double Height { get; set; }
     }
-}
+}