Fix traitlets warning

[omad]

The additional_profile_form_template_paths List traitlet is incorrectly specifying a default value as default=[]. Traitlets now uses default_value as the kwarg instead of default. It could be changed to that, but the default is an empty list anyway.

The warning can be seen at the end of pytest runs, including in GHA, see for example

=============================== warnings summary ===============================
../../../../../opt/hostedtoolcache/Python/3.12.7/x64/lib/python3.12/site-packages/traitlets/traitlets.py:3615
  /opt/hostedtoolcache/Python/3.12.7/x64/lib/python3.12/site-packages/traitlets/traitlets.py:3615: DeprecationWarning: metadata {'default': []} was set from the constructor. With traitlets 4.1, metadata should be set using the .tag() method, e.g., Int().tag(key1='value1', key2='value2')
    super().__init__(trait=trait, default_value=default_value, **kwargs)

[consideRatio]

Thank you @omad!!!