This moves all our JSONField model/form fields to Django's internal classes. These have been available since Django 3.3, and the jsonfield package has been deprecated as of version 3.2.0.

Fixes #9266

We can't actually remove the jsonfield dependency because several0001_* migrations refer to it. After we have merged this PR, we will need to plan another migration reset so we can drop those references. We may want to do this along with or shortly after the Django 5.x migration next year.

Before the migrations can succeed, we need to clean up three Submissions that have null characters in them. These are

>>> Submission.objects.filter(authors__icontains="u0000").values_list("pk", flat=True)
<QuerySet [21133, 21175, 21208]>

These seem to be real cancelled submissions, as the bad data was eventually fixed and the draft submitted successfully. Probably a fixup in the admin to remove the null characters is most appropriate. Testing locally, replacing the \u0000 strings with "(null)" in the authors fields for those drafts allows migrations to succeed. Note that doing this requires #9285 be merged first.

Edit: we can now modify the submissions, but I've also added a step to the migration to fix them automatically. This replaces them with \u2400, which has the virtue of being a single glyph that renders as ␀. This migration protects us against the very unlikely case that another record is created with a null character in it. I checked other places where JSONField is in use - I don't think the others depend on user-generated content in a way that benefits from more thoroughly guarding the migrations.