Use `System.Text.Json` instead of `Newtonsoft.Json` for JSON (de)serialization

[stijnmoreels]

Is your feature request related to a problem? Please describe.
We still use the separate package Newtonsoft.Json for JSON (de)serialization. System.Text.Json is by-default present in the .NET environment.

Describe the solution you'd like
We should consider switching to the Microsoft-package System.Text.Json as default.

Additional context
Discussed in arcus-azure/arcus#171

[stijnmoreels]

Waiting for 'missing members' support: dotnet/runtime#37483

[fgheysels]

Waiting for 'missing members' support: dotnet/runtime#37483

Looks like that issue is fixed.
Apparently, it helps when somebody puts a bit of anger in his voice.

[stijnmoreels]

Hehe, and he's got a point. 😄