[BUG] [openapi json]: requestBody contains $ref and content at the same time

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Json generator can generate both "$ref" param and "content" param.

Config is still valid, but IDEA marks as warning such params. Also swagger-ui couldn't work correctly with such config: https://github.com/springfox/springfox/issues/3956

openapi-generator version

3.5.1 and on latest image on docker is still reproduced

Steps to reproduce

I created minimal demo to reproduce: https://github.com/SButterfly/openapi_json_bug

Expected behavior

I expected only one of "$ref" or "content" to remain.

Start with the minimal reproduction in https://github.com/SButterfly/openapi_json_bug and inspect the generated OpenAPI JSON for requestBody entries. Trace that output into the OpenAPI Generator request-body generation entry point, then run the reproduction to verify that each requestBody contains only one of "$ref" or "content".