feat: upgrade to OpenAPI 3.1.0

[adriangb]

This will have to wait for swagger-api/swagger-ui#5891

[sm-Fifteen]

Changing the default docs UI to something like Stoplight Elements, which has supported OpenAPI 3.1 for some time now, could also be an option.

[adriangb]

Hmm yeah interesting idea, I had never heard of them, I'll take a look. We could also support ReDoc (for unrelated reasons).

My hope is that in the next ~6 months the whole ecosystem (not just Swagger UI, also Pydantic and such) will move to OpenAPI 3.1 & JSON Schema 2020-12. According to this article, these versions will be sort of "long term support" versions. Since it is kinda hard to support multiple versions of these things, I think it makes sense to wait until those versions are stable and then lock into those in Xpresso.

Once we have that, we should be able to easily support multiple UIs, or rather have one out of the box and make it easy to plug in more.