MemcachedCache was deprecated in django 3.2[1], and was later removed
in django 4.1[2].
Because python3-pymemcache package in Ubuntu is too old (2.2.0) and
is not compatible with Django(which requires >=3.4.0), Ubuntu jobs
still use the deprecated python-memcached backend.
[1] 5ce31d6a71
[2] 05f3a6186e
Depends-on: https://review.opendev.org/855298
Change-Id: I10d99c972741b82308b1d4eeddd8cb65f237d6f7
a0bfa42751