Migrate from Newtonsoft.Json to System.Text.Json

[sgtHoloToaster]

It would be good to migrate to System.Text.Json at least for .NET Core 3.1+ compatible versions of the library. But first it's necessary to do some research about how long will it take (mostly depends on whether new versions of Microsoft standard library are dependent on System.Text.Json instead of Newtonsoft.Json). In worst case scenario it could lead to writing own version of JsonPatchDocument which is not so desirable