Upgrade from JSON.NET to System.Text.Json

Development on JSON.NET ended in 2019 and has been mothballed.

System.Text.Json was introduced in Core 2.0 and is a permanent member of .NET 5.

Eventually, we'll need to upgrade to System.Text.Json, but the amount of work and difficulty is high, so creating a ticket for planning. This touches public APIs such as Query, FTS, KV, Analytics and Views as well as internal API's like Config Management and various server negotiation operations.

Note that an important acceptance criteria for this work is that existing users will not be affected, perhaps unless they "opt-in" to any new behavior. This includes any special annotations that may be encountered, etc. Then we can plan a full transition path in the future along with some special communications about what to expect and how to adapt.