perf: make reversion_version table deletion faster (#6493)

### 📣 Summary
Improve background maintenance by making cleanup of the
reversion_version table significantly faster.

Author: noliveleger

kobo/apps/long_running_migrations/jobs/0012_remove_old_versions.py

@@ -15,4 +15,4 @@ def run():
         deleted = queryset.delete()
         # log at debug level so we don't flood the logs
         logging.debug(f'Deleted {deleted[0]} version objects with pk < {min_id}')
-        time.sleep(10)
+        time.sleep(2)

kobo/settings/base.py

@@ -2100,7 +2100,7 @@ def dj_stripe_request_callback_method():
 USER_ASSET_ORG_TRANSFER_BATCH_SIZE = 1000
 SUBMISSION_DELETION_BATCH_SIZE = 1000
 LONG_RUNNING_MIGRATION_BATCH_SIZE = 2000
-VERSION_DELETION_BATCH_SIZE = 1000
+VERSION_DELETION_BATCH_SIZE = 2000
 
 # Number of stuck tasks should be restarted at a time
 MAX_RESTARTED_TASKS = 100