Named category functions/docs need cleaning up

[DrewAPicture]

Spawning from a small discussion in #19766, I think it's about time something is done to deal with the older named category functions such as wp_category_dropdown and wp_list_categories in category-template.php

These functions aren't technically category-only as they accept a set $taxonomy. And because they are actually taxonomy-agnostic, I think we have a few different routes we can take to clean up after ourselves a bit:

A) Lock down these functions to 'categories'-only in favor of new, taxonomy-agnostic ones.

B) Leave the functions alone but let the inline-docs and hard-coded strings such as 'No Categories' reflect their versatility. e.g. 'No Terms' vs 'No Categoriies'.

C) Deprecate these older functions in deference to newer, taxonomy-agnostic ones.