Fix dependency on vulnerable System.Net.Http and System.Security.Cryptography.X509Certificates packages

[fordprefect480]

The current v1.1.1 release depends on vulnerable packages. Recommend updating to reference the latest safe version: v4.3.4.

[GerkinDev]

This repo depends on https://github.com/FubarDevelopment/restsharp.portable which was archived in 2018, that includes lots of outdated packages.

I feel like they just renamed the package and republished it, since sib_api_v3_sdk has the exact same issue.

Also, dup of #1

[WolfgangHG]

I will try to use Kiota and generate the client myself based on https://api.brevo.com/v3/swagger_definition_v3.yml