[BUG] Deprecation Warning in Flask 2.2 (before_first_request)

[RobertJN64]

The new release of Flask 2.2 deprecates the before_first_request method, which results in a warning upon app creation.

dash 2.6.0
flask 2.2.0

Full stack trace:

MLVisualizationTools/dashbackend.py:79: in getDashApp
      app = Dash(__name__, title=title, external_stylesheets=[theme], assets_folder=folder)
  .tox/py38/lib/python3.8/site-packages/dash/dash.py:501: in __init__
      self.init_app()
  .tox/py38/lib/python3.8/site-packages/dash/dash.py:545: in init_app
      self.server.before_first_request(self._setup_server)
  .tox/py38/lib/python3.8/site-packages/flask/scaffold.py:[49](https://github.com/RobertJN64/MLVisualizationTools/runs/7633737039?check_suite_focus=true#step:5:50): in wrapper_func
      return f(self, *args, **kwargs)
  _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
  
  self = <Flask 'MLVisualizationTools.dashbackend'>
  f = <bound method Dash._setup_server of <dash.dash.Dash object at 0x7f27a07a00[70](https://github.com/RobertJN64/MLVisualizationTools/runs/7633737039?check_suite_focus=true#step:5:71)>>
  
      @setupmethod
      def before_first_request(self, f: T_before_first_request) -> T_before_first_request:
          """Registers a function to be run before the first request to this
          instance of the application.
      
          The function will be called without any arguments and its return
          value is ignored.
      
          .. deprecated:: 2.2
              Will be removed in Flask 2.3. Run setup code when creating
              the application instead.
      
          .. versionadded:: 0.8
          """
          import warnings
      
  >       warnings.warn(
              "'before_first_request' is deprecated and will be removed"
              " in Flask 2.3. Run setup code while creating the"
              " application instead.",
              DeprecationWarning,
              stacklevel=2,
          )
  E       DeprecationWarning: 'before_first_request' is deprecated and will be removed in Flask 2.3. Run setup code while creating the application instead.

[RobertJN64]

I am going to mark this as closed because it is fixed in the plotly:dev branch.