Migrate to libopenapi to support OpenAPI 3.1+

I would like to use protoc-gen-openapi but I need to use OpenAPI 3.1.
Current, this is using kin which is only (ever) going to support OpenAPI 3.0

https://github.com/pb33f/libopenapi/ supports OpenAPI 3.0, 3.1 and swagger, and is well maintained. 

I would like to propose moving this project to rely on https://github.com/pb33f/libopenapi so it can support OpenAPI 3.1