Invalid properties included in OpenAPI v3.0 output document

[Azquelt]

If output in OpenAPI v3.0 is configured, properties which were added in OpenAPI 3.1 are included in the output document.

Previously, if the output mode was 3.0, we would drop any properties which are not valid for 3.0, I think that changed in #2021.

I don't think OpenAPI allows the addition of arbitrary fields, so I think we should revert this behaviour change to ensure we're always outputting a valid OpenAPI v3.0 document.