API spec version wrong

The openapi spec seems to declare nullable properties in a manner consistent with the 3.0.0 spec and not the 3.1.0 spec.

This leads to some generators generating wrong code (ignoring the nullability of properties) and causing generated clients to error, when parsing api responses.

Trying to use the spec with the 3.1.0 version generates wrong code:

https://github.com/OpenAPITools/openapi-generator/issues/24108