reconfirm trackProximity zoom threshold is correct

Nobody has claimed this yet.

Currently the trackProximity option will only send the proximity to the Geocoding API at zooms > 9, this is based on proximity being designed for neighbourhood scale so isn't helpful to send when the map is zoomed out.

It appears the logic in the Mapbox Geocoding API may have changed since this was done

https://github.com/mapbox/carmen/blob/0015a66dab6ba8a927cde06b744cb9991f15bd45/lib/util/proximity.js#L116-L150

Although there is still a gaussian curve used to give much more weightings nearby the proximity than away from the proximity, the scale of the curve varies based on type.

country, region based on z6
district, place, locality, postcode, neighborhood based on z12
poi, address based on z14

However since the Geocoding API doesn't support passing an accuracy radius with the proximity coordinates we still should retain some zoom limit to sending the proximity.

I'm opening this ticket so that the geocoding team can weigh in on if the assumptions here about only sending proximity at zoom>9 make sense or if it should be tweaked.

this addresses one of the issues raised in https://github.com/mapbox/mapbox-gl-geocoder/pull/320

Start by reviewing the trackProximity zoom threshold and the referenced carmen proximity.js behavior, then compare the current assumptions with the Geocoding API behavior described in the issue. Done means the geocoding team has decided whether the zoom >9 limit remains appropriate or should be changed.

Written by the indexing model from the issue text.