Fixes after restsharp update

[sanctusmob]

Summary of Changes

This pull request addresses several issues and makes enhancements for compatibility and functionality:

Fixed Project Compatibility Issues:
    Resolved errors preventing the project from opening due to .NET version incompatibilities:
        Updated the TargetFramework from netcoreapp2.1 to netcoreapp3.1 or later, resolving issues with System.Runtime.CompilerServices.Unsafe (issue #475).
        Addressed compatibility between PrestaSharp and .NETFramework, Version=v4.5.2.

Fixed Deserialization Issue with Product Accessories:
    Fixed deserialization errors occurring when a product with accessories is deserialized (Sequence contains more than one element). This was due to confusion between the root "product" element and the "product" elements in accessories (issue #476).

Enhancements:
    Added round_mode and round_type fields to the Order entity.
    Corrected the singular name of the customization field.

Testing and Compatibility

These changes have been thoroughly tested for over 3 months in PrestaShop versions 1.7.8 and 8.1.7, ensuring reliability and stability.