Add multi-targeting

[kudima03]

Update the project to support multiple .NET target frameworks so the NuGet package can be used in different runtime environments.

Tasks:

• Replace <TargetFramework> with <TargetFrameworks> in the .csproj file.
• Add all required target frameworks (e.g. net7.0;net8.0;net9.0).
• Ensure the project builds successfully for all targets.
• Verify that the generated NuGet package includes assemblies for each framework.