Use System.Text.Json instead of Newtonsoft.Json

[szelpe]

https://www.nuget.org/packages/System.Text.Json

[martonx]

Hey, I finally finished the forked Barion .Net SDK v2. https://github.com/martonx/barion-dotnet-v2
It used System.Text.Json instead of old unnecessary NewtonSoft.Json lib.