Update celery dependency to v5.x

Celery v5 has entered unstable and could migrate to testing before bullseye release. We need to update debexpo accordingly.

TODO:

• Update setup.py dependency
• Update python3-django-celery-beat to v2.1 (otherwise incompatible with celery 5, see #972565)
• Convert deprecated @periodic_task to the add_periodic_task method