chore: Replace deprecated django.contrib.postgres.forms.JSONField with django.forms.JSONField

Part-of: <https://dev.funkwhale.audio/funkwhale/funkwhale/-/merge_requests/2709>

api/funkwhale_api/common/preferences.py

@@ -2,7 +2,7 @@ import json
 
 from django import forms
 from django.conf import settings
-from django.contrib.postgres.forms import JSONField
+from django.forms import JSONField
 from dynamic_preferences import serializers, types
 from dynamic_preferences.registries import global_preferences_registry