Right-size Redis Persistent after Redis Sessions is live and stable

Redis sessions is expected to remove something in the order of 70% of data storage from Redis persistent, and since production#5546 that node has been quite large (120GB RAM) for additional safety margins, while only using around 10% of that for core Redis data storage. AFter this work, that number will drop significantly (<5%?)

Therefore once we have completed the migration of session data to the new Redis and will not be rolling back, we should re-evaluate the sizing of Redis Persistent.

At a minimum we should expect to halve the instance size (we will have removed more than half the data), but could consider going even lower, based on concrete observations.

It will be important to give the persistent Redis some time to stabilize and possibly compact memory a bit, after migrated session data has expired/been deleted. 1 week would be a minimum, and it may take longer and we could also force a failover and observe the effective memory usage of the resync'd replica which will be freshly compacted/rebuilt.