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.PdfEditor/EnvelopeGenerator.PdfEditor.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net462;net7.0;net8.0;net9.0</TargetFrameworks>
+    <TargetFrameworks>net462;net8.0-windows</TargetFrameworks>
   </PropertyGroup>
 	
   <PropertyGroup Condition="'$(TargetFramework)' == 'net462'">
@@ -17,9 +17,8 @@
   </PropertyGroup>
 	
   <ItemGroup>
-	<PackageReference Include="BouncyCastle.Cryptography" Version="2.6.2" />
-	<PackageReference Include="itext" Version="8.0.5" />
-	<PackageReference Include="itext.bouncy-castle-adapter" Version="8.0.5" />
+	<PackageReference Include="GdPicture" Version="14.4.7" />
+	<PackageReference Include="GdPicture.runtimes.windows" Version="14.4.7" />
   </ItemGroup>
 	
   <ItemGroup>