Migrate to OpenAPI 3.x.x

[Zapata]

OpenAPI 3.x.x is now the standard, Swagger 2.x is deprecated.

[Zapata]

Note that zalando/connexion already handles both, so we should only have to update the yaml files.

Move also to Swagger UI 3.xxx.

[Zapata]

Try this converter: https://mermade.org.uk/openapi-converter