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.Web/EnvelopeGenerator.Web.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk.Web">
 
 	<PropertyGroup>
-		<TargetFrameworks>net7.0;net8.0;net9.0</TargetFrameworks>
+		<TargetFrameworks>net8.0;net9.0</TargetFrameworks>
 		<Nullable>enable</Nullable>
 		<ImplicitUsings>enable</ImplicitUsings>
 		<PackageId>EnvelopeGenerator.Web</PackageId>
@@ -2169,6 +2169,7 @@
 
 	<ItemGroup>
 	  <PackageReference Include="BouncyCastle.Cryptography" Version="2.6.2" />
+	  <PackageReference Include="Microsoft.Bcl.AsyncInterfaces" Version="10.0.11" />
 	  <PackageReference Include="Microsoft.Identity.Client" Version="4.82.1" />
 	  <PackageReference Include="SixLabors.ImageSharp" Version="3.1.12" />
 	</ItemGroup>