Only support OpenAPI 3.1

[steve-chavez]

The roadmap mentions:

The first step in the roadmap is to migrate the OpenAPI spec from the PostgREST core repository (version 2.0 to 3.1):

https://github.com/PostgREST/postgrest-openapi#roadmap

Originally we thought it would be possible to replace the core openapi output in Haskell by somehow inlining the functions of this extension into a single query. But that seems too difficult (maybe impossible).

So I think we can just focus on openapi 3.0, to reduce the scope of the project.

With this we should still be able to fix all postgrest core issues. Since this lib is pure SQL, we can just instruct users to install it. They have been doing that for pgjwt for a long time anyway.

[wayland]

I've been focussing on 3.1.0.