Upgrade to OpenAPI 3.2.0

[opqdonut]

It looks like we could just bump the version number, since 3.2.0 is backwards-compatible. However, we could also consider using the new features like hierarchical tags.

Probably not urgent, users can also override the version number themselves using

["/openapi.json"
 {:get {:openapi {:version "3.2.0"}
        :handler (openapi/create-openapi-handler)}}]

https://learn.openapis.org/upgrading/v3.1-to-v3.2.html