In GitLab v12.2.0, we enabled the Rails Redis caching store in
production
(https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30966). However,
we had to to revert this in 6bda359b
because Rack Attack v4.4.1 wasn't fully compatible with this store and
caused fatal errors when rate limits were enabled
(https://github.com/kickstarter/rack-attack/issues/281).

Now that we have upgraded to Rack Attack v6.2.0, which pulls in
https://github.com/kickstarter/rack-attack/pull/340, this caching store
is now supported. This is validated by the GitLab QA tests introduced
in https://gitlab.com/gitlab-org/gitlab-foss/merge_requests/32177.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/64794