bugfix: exclude 406 status code violations

[pboos]

Excludes violations similar to this one:

OpenAPI spec validation error [validation.response.status.unknown]
GET https://api.example.com/users
Response Status Code: 406

INFO - Response status 406 not defined for path '/users'.: []

Reason
If the service returns a 406 this is the expected response if a client asks for something the service can't deliver.