Change `horizon` in `step_growth_rate()` and `arx_class_args_list()`

[rachlobay]

We sometimes use horizon interchangeably with ahead and so perhaps we should change the name of it in step_growth_rate() and arx_class_args_list() (and anywhere else it gets used in such a way in this package). Since it is passed to the h argument of epiprocess::growth_rate(), then maybe the easiest thing is to just refer to it as h?

[brookslogan]

Since horizon is used by the Hubverse for "reference date"-relative targeting (which are something like a label for (the end of) some time interval containing the forecast submission deadline), perhaps we want to stay away from that term and h for these, and standardize around ahead, at least for max time_value-relative stuff?

(The reasoning behind these reference dates includes allowing some flexibility for forecast deadlines to change without changing horizon-labeling and without having people have to be aware of when to pretend forecasts were submitted on a different date. E.g., the time intervals used were epiweeks, so changing deadline from say Tue -> Wed would not impact the reference date.)