MT#56231 workaround for django-celery-beat issue

* https://github.com/celery/django-celery-beat/issues/647
* https://github.com/Salamek/cron-descriptor/issues/65
* spotted solution at
  82f1370155

Change-Id: I2da6f0f65050d7c7b97609c0505c4647915d52c7

repoapi/settings/prod.py

@@ -153,12 +153,12 @@ CELERY_BEAT_SCHEDULE = {
     # Executes every Sunday morning at 7:30 A.M
     "purge-trunk": {
         "task": "repoapi.tasks.jbi_purge",
-        "schedule": crontab(hour=7, minute=30, day_of_week="sunday"),
+        "schedule": crontab(hour=7, minute=30, day_of_week="sun"),
         "args": ("none", 4),
     },
     "purge-none": {
         "task": "repoapi.tasks.jbi_purge",
-        "schedule": crontab(hour=7, minute=30, day_of_week="sunday"),
+        "schedule": crontab(hour=7, minute=30, day_of_week="sun"),
         "args": (None, 1),
     },
     "gerrit-cleanup": {