Drop support for Zen Cart versions prior to 1.5.8

[lat9]

Primarily in support of the orders_status page's changes. It's unclear to me whether the downloads' processing will work with that version.

Essentially, a reduction in the number of Zen Cart versions which require 'back-testing'.

[lat9]

Changing milestone, since (at this point), the various changes should still work with zc157a-d and some of the changes are significant!