[OAS 3.1.0] In License object url and identifier should not be both set

[jmini]

With OpenAPI 3.1.0 there is an new identifier to the License object.

Reference: https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.1.0.md#licenseObject

An SPDX license expression for the API. The identifier field is mutually exclusive of the url field.

Invalid example:

openapi: 3.1.0
info:
  title: Example specification
  version: "1.0"
  license:
    name: Eclipse Public License 2.0
    url: https://www.eclipse.org/legal/epl-2.0/
    identifier: EPL-2.0
servers:
- url: http://localhost:8080/
paths:
  /ping:
    get:
      operationId: ping
      responses:
        200:
          description: OK