The CI should test against our lowest supported Redis version

As of 2020-08, it uses redis:alpine but this should instead be pinned to the oldest version we support, which as of %13.0 is 4.

(@tnir EDITED) As of 2021-04, the CI uses redis:5 (#238216 (comment 495055423)) but this should instead be pinned to the oldest version we support, which as of %13.0 is 4.