django 4 support: Adapt production settings for memcached changes

This adapts the memcached import the same way the swh.web module did.

Follow up on 4698e8c9

I currently have issues on pod otherwise:

do-migration     raise InvalidCacheBackendError(
do-migration django.core.cache.backends.base.InvalidCacheBackendError: Could not find backend 'django.core.cache.backends.memcached.MemcachedCache': Module "django.core.cache.backends.memcached" does not define a "MemcachedCache" attribute
/class

Refs. swh/infra/sysadm-environment#5129 (closed) [1]