feat: migrate from Newtonsoft.Json to System.Text.Json

[paviad]

Fixes

Closes #1208

Possibly impacts #985 (which is a duplicate of a #1050 which is a request to support System.Text.Json)

Description

This PR includes conditional compilation checks to use System.Text.Json instead of Newtonsoft.Json everywhere when targeting .NET Standard 2.0

The tests projects have been duplicated and modified to test both the old and the new versions.

All tests pass (except integration tests, which I could not get to work despite having configured a valid SendGrid API Key) - As a consequence of this, all integration tests are marked for skipping using [Fact(Skip = ... and [Theory(Skip = ...

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
• I have updated my branch with the main branch
• I have added tests that prove my fix is effective or that my feature works
• I have added the necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified

If you have questions, please file a support ticket, or create a GitHub Issue in this repository.