text: Title: @root_validator will not updated to model_validator
  Body: ## Description
  I have a @root_validator without having any parameter in my project like this:
  ```
  from pydantic import BaseModel, root_validator
  
  class Model(BaseModel):
      name: str
  
      @root_validator
      def validate_root(cls, values):
          return values
  
  ```
  In this way, `bump-pydantic` can't detect it and will not update it to a new version and without any TODO or anything. 
  Also, if I define my root_validator like `@root_validator()`, it will be detected and converted to `@model_validator()` without defining `mode` parameter.
  The only successful convert will happen if I have `@root_validator(pre=True)`. I've not checked other scenarios.
  
  If it is an accepted bug, I will be happy to work on it and make a PR
  
  ## My Environment
  Python 3.11.4
  bump-pydantic==0.7.0