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.
Upgraded BouncyCastle.Cryptography from 2.5.0 to 2.6.2 across all projects, updating project and package config files as needed. Added/updated PackageReference for .NET Core/Standard projects. Updated app.config with a new binding redirect for System.ValueTuple and changed Microsoft.Identity.Client redirect to 4.55.0.0. These changes improve dependency consistency and resolve potential assembly binding issues.
