When I implemented the migration code to update the CACHES setting from the old `MemcachedCache` to the new `PymemcacheCache`, I tested the actual migration implementation, but had just made an assumption about the code that checks if we need to run it. It turns out this had two problems--we were skipping one step that needed to check if a settings update was required, and in the running context of rb-site, the key that we need to check is `forwarded_backend` instead of `default`. Testing Done: Ran a real `rb-site upgrade` and saw that my settings_local.py file was correctly updated.