Simplify native's `ActiveIn` behaviour

[AnnaShaleva]

Native deploy and update code becomes more complicated after #3444. A lot of problems causes nullable native's ActiveIn: https://github.com/nspcc-dev/neo-go/blob/2d4993a8375121a5bf073eb37e4dd92451a89f27/pkg/core/interop/context.go#L175-L177

If we use config.HFDefault instead of null ActiveIn and make ActiveIn to return config.Hardfork instead of *config.Hardfork, it will simplify things a lot and allow to unify native deploy/update code.