Switch serializer from Newtonsoft.Json to System.Text.Json

[Cheesebaron]

System.Text.Json is the new defacto standard for serialization in .NET and in many cases much more memory efficient and faster.
Especially when enabling source generated serializing.

Another reason would be to not have to pull in another transitive dependency as part of the package as System.Text.Json is available for most TFMs out of the box.