Replace DeepEquals and DeepClone with new System.Text.Json APIs

[weichch]

Once the two APIs are available in newer version of System.Text.Json, the two custom made methods should be decommissioned (if possible).

Track progress in dotnet/runtime#56592.
Also related this dotnet/runtime#33388.

[weichch]

A few changes has been added to version 2.0.0 following the resolution of dotnet/runtime#56592.