MemcachedCache backend was removed in django 4.1[1]. The latest horizon
requires django>=4.2[2] so we are no longer able to use the backend.
[1] 05f3a6186e
[2] 605a515ebc31beb66ff46cb193bc96c62d086ff6
Change-Id: Ia8c3f11aa33d4a48577be83c3372229818789543