Deprecate gradient

Posted under Tags

BUR #14323 has been approved by @evazion.

deprecate gradient
mass update gradient gradient_* -> -gradient

The situation surrounding gradient is essentially identical to that of shiny, which was recently deprecated in topic #22667. It is a 100k-strong tag only off of the backs of its implications - once again, barely 5000 posts exist that aren't the result of an implication, of which a single one (gradient_background) is responsible for over 90% of gradient's count.