Update Celery config to use UTC time

[whabanks]

Summary | Résumé

This PR updates the Celery and Celery beat configs to enable and use UTC time instead of EST / local time.

Testing

Do this off main first, or change the celery timezone config back to America/Toronto temporarily.

1. Set a distinctive NOTIFICATION_QUEUE_PREFIX in your .env file
2. Run the app + Celery + beat
3. Schedule several bulk notifications to yourself in the future, 5 or 10 minutes will do
4. Shut down Celery + beat
5. In staging SQS, check the <queue-prefix>-bulk-tasks and <queue-prefix>-periodic-tasks queues and verify that the messages are present

Part 2

1. Check out this branch or restore the timezone config back to UTC
2. Run Celery + beat
3. Receive your scheduled notifications
4. Observe the the queues being drained of tasks/messages, that were scheduled while Celery was configured to use EST time, without issue when configured to use UTC time.

need to test

[jzbahrai]

Tested by setting a job in the future, but then changing to this branch - the job completed!