Allow 4.0 to generate OpenAPI v3.0 as well as OpenAPI v3.1

[Azquelt]

At the moment, the main-4.0 branch only generates OpenAPI 3.1 documents.

We would like it to also be able to generate OpenAPI 3.0 documents.

I think we could do this in the (de-)serialization step, doing a transformation from 3.1 <-> 3.0 where necessary.

Testing effort required for this could be quite large.