Migrate from Newtonsoft to System.Text.Json

[thompson-tomo]

With release and evolution of System.Text.Json the json serialisation should be switched to STJ so that the framework can natively provide the library on newer frameworks & at the same time reduce maintenance.

[vaclav-antosik]

+1 for this one. This change is highly expected.

[thompson-tomo]

Agree & as such PR has been done which Just needs review/release

[pepki]

Support for this.
Adding to what has been said, Newtonsoft 9.0.1 also has a vulnerability with high severity https://www.nuget.org/packages/newtonsoft.json/9.0.1