Consider renaming designKits schema and "Design kits" catalog to "Design libraries"

[mattrosno]

Summary

What problem (or opportunity) are we solving? Why are we solving it? Is there supporting user
conversations or research?

Originating from this PR discussion: #1515 (comment)

It's confusing that we include the word "library" when describing design kits, yet those design kits don't show up in the libraries catalog.

Upon realization that Sketch, Figma, and Adobe UX natively use the word "libraries", and Axure uses "widget libraries"... what if we drop structured use of design kit and went with "Design library":

This would likely require a breaking schema change so it'd be more than just a web-app change. But as @aubrey-oneal said, there's something nice about the symmetry here.

Job stories

What value will this deliver?

1. When _, I want to _, so I can _.

Acceptance criteria

What requirements must be met to complete this request?

1. To do