Migrating to System.Text.Json

[just-ero]

Description

As mentioned by @yufeih in #9717 (reply in thread), the use of NewtonSoft.Json in this library is due to the age of it. That's understandable.

However, using .NET's own System.Text.Json namespace should provide at least these obvious benefits:

• Clear performance advantage.
• One less third-party dependency.

I propose that a gradual move to System.Text.Json be made, despite it not being trivial. This move can coincide with the change to providing public config/options classes proposed in #9726.

Feel free to close if not planned or move to a "Backlog"-type milestone.

[yufeih]

Thank you @just-ero for offering help! This is definitely something we are willing to accept as a contribution. We appreciate your support.