Switch to the upstream redis chart

Summary

Drop our old outdated redis chart in favour of the upstream, bitnami maintained chart. https://github.com/helm/charts/blob/master/stable/redis

Benefits of using upstream

1. Remove redis and redis-ha forks
2. No undocumented behavior when redis & redis-ha charts are both enabled somehow
3. Reduced load on our team for maintenance
4. Now a StatefulSet, being more resilient

Context

Prior research on how to go about doing this was completed here: https://gitlab.com/charts/gitlab/issues/1221

The hope is to replace both the redis-ha and the redis chart with this new chart.

• For now replacing the redis-ha chart is blocked on Adding sentinel support to the rest of the chart: https://gitlab.com/charts/gitlab/issues/1501
• Replacing the redis chart is blocked some upstream MRs:
  • https://github.com/helm/charts/pull/16363
  • https://github.com/helm/charts/pull/16360

Upgrade concerns

My conclusion from the research in https://gitlab.com/charts/gitlab/issues/1221#note_203777943 is that it's possible to upgrade users smoothly to the new chart, but requires us to vendor the chart to persist the existing volume claim. And due to the low-importance of the data stored in redis, that we should instead start fresh if we do the upgrade during a Major release of the chart. (And not need to vendor the new chart at all)

We are planning to do a Major release for our postgres upgrade: https://gitlab.com/charts/gitlab/issues/928 So we should schedule this switch for the same time.