Move to System.Text.Json

[idg10]

We want to move off Newtonsoft.Json and onto the System.Text.Json features built into .NET.

There are a few reasons for this:

• simplifying dependencies
• System.Text.Json is typically much faster
• System.Text.Json offers the possibility of much less GC overhead
• it gets us closer to Corvus.JsonSchema

Our goal is to enable services to use types generated by the tooling in Corvus.JsonSchema for serialization, deserialization, and validation. That's where #309 is trying to get to.

Since Corvus.JsonSchema is based entirely around System.Text.Json, moving away from Newtonsoft.Json is necessary first step.