The django-multiselectfield package hasn't been maintained in a few

years, and it doesn't support Django 4.x+. There's a third-party fork

which has had some updates, but no releases that we can consume.
django-multiselectfield itself isn't very big, and of what it offers, we

don't actually use very much. A new field has been added to Djblets to

handle the list serialization/deserialization for comma-separated

values. This change switches us over to that.
The major difference here is that we have to instantiate the form

field ourselves. While the old MultiSelectField had its own validation

for the database side, we can rely on the form's MultipleChoiceField

validation.
This also manually sets a max_length on the field. The MultiSelectField

would compute the max_length based on the provided choices, but that

meant that any change to the choices would need an evolution.

Ran unit tests.
Ran the new evolution.
Tested the WebHookTarget admin form and verified that everything

  worked as expected.