♻️ refactor: Newtonsoft.Json to System.Text.Json

[Stoom]

Overview

This swaps out Newtonsoft.Json for the core library's System.Text.Json implementation.

Breaking changes

• System.Text.Json does not read class attributes when inheriting a class. This means you'll need to add the [JsonConverter(typeof(SingleValueObjectJsonConverter))] attribute to any class that derives from SingleValueObject. One alternative would be to register a default json config and pull when setting up a serializer.