In OpenAPI 3.0.x, Cannot use null type

[takuya-ebata]

In OpenAPI 3.0.x, we should use nullable property instead of use null type,

https://swagger.io/docs/specification/data-models/data-types/

however, In OpenAPI 3.1, we should use null type instead of nullable
OAI/OpenAPI-Specification#2244

So that, I want to know which is better opinion in spec-tools

• we support OpenAPI 3.0.x (we only support nullable)
• we support OpenAPI 3.1 ~ (we only support null)
• we support 3.0.x and 3.1 with new namespace like openapi31, openapi30
• user should change nullable <-> null using some functions.

ref: https://github.com/metosin/spec-tools/blob/master/docs/06_openapi.md

If you give me advice, I'll write some PR if I can.