Fix celery settings warnings

Celery on start output:

[2021-10-10 13:45:59,114: WARNING/MainProcess] /usr/lib/python3/dist-packages/celery/app/utils.py:204: CDeprecationWarning:
    The 'CELERY_RESULT_SERIALIZER' setting is deprecated and scheduled for removal in
    version 6.0.0. Use the result_serializer instead
  deprecated.warn(description=f'The {setting!r} setting',
[2021-10-10 13:45:59,114: WARNING/MainProcess] /usr/lib/python3/dist-packages/celery/app/utils.py:204: CDeprecationWarning:
    The 'CELERY_TASK_SERIALIZER' setting is deprecated and scheduled for removal in
    version 6.0.0. Use the task_serializer instead
  deprecated.warn(description=f'The {setting!r} setting',
[2021-10-10 13:45:59,114: WARNING/MainProcess] /usr/lib/python3/dist-packages/celery/app/utils.py:204: CDeprecationWarning:
    The 'CELERY_RESULT_BACKEND' setting is deprecated and scheduled for removal in
    version 6.0.0. Use the result_backend instead
  deprecated.warn(description=f'The {setting!r} setting',
[2021-10-10 13:45:59,114: WARNING/MainProcess] /usr/lib/python3/dist-packages/celery/app/utils.py:204: CDeprecationWarning:
    The 'CELERY_ACCEPT_CONTENT' setting is deprecated and scheduled for removal in
    version 6.0.0. Use the accept_content instead
  deprecated.warn(description=f'The {setting!r} setting',
[2021-10-10 13:45:59,114: WARNING/MainProcess] /usr/lib/python3/dist-packages/celery/app/utils.py:204: CDeprecationWarning:
    The 'CELERY_TIMEZONE' setting is deprecated and scheduled for removal in
    version 6.0.0. Use the timezone instead
  deprecated.warn(description=f'The {setting!r} setting',
[2021-10-10 13:45:59,114: WARNING/MainProcess] Please run `celery upgrade settings path/to/settings.py` to avoid these warnings and to allow a smoother upgrade

To fix (I suspect using the dict settings dedicated to celery)