Add additional TFM to enable dependency optimisation

[thompson-tomo]

Is your feature request related to a problem? Please describe.
I want to minimise dependencies in my project by utilising framework dependencies wherever possible

Describe the solution you'd like
I want the package to not have an explicit dependency on System.Text.Json as it can be provided by the framework when targeting net 6.

Describe alternatives you've considered
Accept the additional dependency

Additional context
n/a