Switch to OAPI for codegen

[mattoni]

Switching this for a few reasons

• More idiomatic go, and a lot less magic that was happening in ogen
• As a result of less magic, there are less esoteric errors when generating code from the spec that may not work with the magic. Discriminated unions in ogen are picky, not as much here.
• Seems this library is most likely to be supporting 3.1.0 first, open discussions on adopting libopenapi under the hood which should help with future versions too.

Still need to downconvert the OpenAPI spec for now, but it's acceptable for now.