[api-diff] change classification of remove-enum/add-enum-value in response

[iugaidiana]

It is needed to change classification for the following cases in response schema - paths.[].[].responses.[].content.[].schema:

1. add new enum value
   • changeId: add-enum-value
   • severity: requires attention
     Example:

before:

openapi: 3.0.0
...
paths:
  /pets:
    get: 
      responses:
        '200':
          content:
            'application/json':
                schema:       
                   type: string
                   enum:
                     - val1
                     - val2

after:

openapi: 3.0.0
...
paths:
  /pets:
    get: 
      responses:
        '200':
          content:
            'application/json':
                schema:       
                   type: string
                   enum:
                     - val1
                     - val2
                     - val3

---

2. remove entire enum with all values
   • changeId: remove-enum
   • severity: requires attention
     Example:

before:

openapi: 3.0.0
...
paths:
  /pets:
    get: 
      responses:
        '200':
          content:
            'application/json':
                schema:       
                   type: string
                   enum:
                     - val1
                     - val2

after:

openapi: 3.0.0
...
paths:
  /pets:
    get: 
      responses:
        '200':
          content:
            'application/json':
                schema:       
                   type: string

tbd: separate ticket to change description for risky changes

[b41ex]

@iugaidiana let us consider also similar cases when constraints are removed from string properties (e.g. format is removed). Probably classification should be the same for consistency.