Use lowest supportable dependency versions

In the interest of maximising compatibility I have lower the required version of System.Text.Json the only dependency for this library to the lowest supportable version I can, 6.0.0.

This should increase the places this library can be used, such as within function apps, and will be my strategy going forwards. Limit the number of external dependencies (zero is the best number) and if I need to take one on, use its lowest supportable version.