Update to use OpenAPIKit 4.0

[heckj]

Motivation

While trying to generate types for DocC, which has multiple OpenAPI spec files, I ran across #25, which then led to #821, after which a holiday nerd-snipe echoed in my head "How hard could this be?

Modifications

• updated dependency to OpenAPIKit 4.0
• Updated tests that use slightly new phrasing ("Problem" instead of "Inconsistency")

Result

• (WIP) there's still a unit test that isn't passing, but I need to do more research to understand what's intended to happen here and how that delta is impacted by OpenAPIKit 4.x

Test Plan

• unit tests
• seeing what CI shows here