fix #1528 django-constance: can't disable boolean setting

[titusz]

The issue occurs because Django Unfold's constance integration uses django.forms.BooleanField with the default
configuration, which has required=True by default. In Django's BooleanField:

• required=True means the checkbox MUST be checked for the form to validate
• required=False means the checkbox can be either checked (True) or unchecked (False/None)

When a user tries to uncheck a boolean setting in django-constance through Unfold's interface, the form validation
fails because the BooleanField expects the checkbox to be checked (since required=True).