Flurl.Http 4.0.0-pre1

• Drop dependency on Newtonsoft.Json in favor of System.Text.Json (#517)*
• Drop non-generic dynamic-returning JSON methods (#699)
• Update target frameworks (#544 )

*WARNING: Your code may fail in subtle ways after upgrading if it depends on things like serialization attributes or global settings from the Newtonsoft library. To continue to use Newtonsoft with Flurl.Http 4.0 and later, add and register this Newtonsoft serializer in your code.