migrations: Disable atomic flag on migration 0354.

I believe that this migration with the default of atomic=True will
fail when trying to convert the field to PositiveIntegerField if there
were any 0 values present in the database when the migration began.
The fix is to have each of the steps be their own transaction.

zerver/migrations/0354_alter_realm_message_content_delete_limit_seconds.py

@@ -34,6 +34,7 @@ def reverse_make_zero_invalid_for_message_delete_limit(
 
 
 class Migration(migrations.Migration):
+    atomic = False
 
     dependencies = [
         ("zerver", "0353_remove_realm_default_twenty_four_hour_time"),