Update to System.Text.Json

[michael-hawker]

Is your feature request related to a problem? Please describe.
Increase performance and stay up-to-date by moving to the System.Text.Json package. Info here.

Describe the solution you'd like
Remove Newtonsoft.Json package and update to System.Text.Json.

[madsbangh]

+1 on this.

snyk is warning about a vulnerability in the version of Newtonsoft.Json currently beiing used. It would be nice to at least get rid of that warning:

[dgkanatsios]

@madsbangh we did the fix in the PR, but there are some challenges with uploading the changes to Nuget unfortunately. Till we fix them, you can reference or build the source code directly, apologies for the inconvenience.