support openapi 3.1.0

Proposal

Support for openapi 3.1.0.

Currently, openapi 2.x and 3.0.x are supported, meaning that when adds an openapi spec with that version, gitlab shows it in a swagger-ui interface like this:

However, when using openapi 3.1.0, this is the result on the gitlab web client:

It seems swagger-ui is used, and only release v4.x of swagger-ui will support openapi 3.1.0, and it seems swagger-ui 4.0 isn't out yet: https://github.com/swagger-api/swagger-ui/tree/release/4.x . An alternative to swagger UI may be:

• https://github.com/Redocly/redoc (basic 3.1.0 support)
• https://github.com/mermade/openapi-gui (not sure about 3.1.0 support)