Move library from Newtonsoft.Json to System.Text.Json

Microsoft & many developers have been moving their apps & libraries from `Newtsoft.Json` to `System.Text.Json` recently (e.g. the Microsoft Graph SDK for .NET).
It would be very appreciated if this library could be moved as well. Not only is `System.Text.Json` faster and more efficient, it is also already used by many developers and libraries. `Newtonsoft.Json` isn't exactly the smallest .dll and increases the binaries of apps we are creating with the Google API Library for .NET.

This suggestion has been posted by @LindaLawton in https://github.com/googleapis/google-api-dotnet-client/issues/1464#issuecomment-945831437 before and I have read the discussion following on it but I was afraid that this doesn't get the visibility needed when not posted as an own feature request.