Apply ElastiCache service upgrades

# Summary
Our Redis clusters have service upgrades available to apply. Each upgrade can take between 15 - 30 minutes to apply, during which time the cluster nodes are upgraded sequentially. As a result, there can be a few seconds of downtime as each node is upgraded.

From the FAQ:

> Q: What is the impact of applying a service update to ElastiCache clusters? Will I lose data or connectivity to my clusters?
>
> When you or Amazon ElastiCache applies a service update to one or more clusters, the update is applied to no more than one node at a time within each shard until all selected clusters are updated. The nodes being updated will experience downtime of few seconds, while the rest of the cluster will continue to serve traffic.

We should apply the upgrades to Staging and monitor for service impact so that we can plan the Production upgrade.

# Related
- https://aws.amazon.com/elasticache/elasticache-maintenance/

Start with the AWS ElastiCache service-maintenance FAQ linked in the issue and inspect the staging Redis clusters and their available upgrades. Apply the upgrades in Staging, monitor for downtime or other service impact, and document the results before planning the Production upgrade.

Written by the indexing model from the issue text.