Use `System.Text.Json` instead of `Newtonsoft`

[roxk]

.NET 8 will soon be released. System.Text.Json had gained most of the capability of Newtonsoft, and the ecosystem as a whole has embraced and migrated to STJ. Please consider using STJ instead of Newtonsoft so apps and libraries that use STJ doesn't get impedance mismatch with contentful's csharp SDK (e.g. when interfacing with JsonObject type, which is currently exposed as a newtonsoft type instead of STJ's type).

[Roblinde]

@roxk been considering this for some time. It will be quite a bit of work and might require some larger refactorings.