Advancement box textures use nine-slice scaling, but seemingly cannot be configured

Discovered while testing MC-269751.

The bug

The textures box_obtained and box_unobtained.png, from textures/gui/sprites/advancements, use what I assume must be nine-slice scaling to accommodate different advancement name lengths. However, this scaling does not appear to be controlled via json files, unlike all other UI sprites which use a scaling system such as buttons, sliders and scroll bars, and adding in your own json files for them causes strange, unexpected graphical results.

How to fix

Get rid of the hardcoded scaling behaviour, switch it over to the json control that all other scaled UI textures use.