`default_app_config` is deprecated in Django 3.2 and should be removed

[nattyg93]

default_app_config was deprecated in Django 3.2 . Given this project now only supports a minimum of Django 3.2 the default_app_config should be removed.

Note, I came across this because Django was emitting a warning. I was able to silence this warning by changing my INSTALLED_APPS from "herald" to "herald.apps.HeraldConfig".