govuk-aws: 29. Combine api-redis into backend-redis
We currently have 3 Redis instances that we run in Elasticache:
Most applications use backend-redis, the logging cluster uses logs-redis (which should soon be replaced), and Rummager uses api-redis.
This was traditionally the case because Rummager lived in a different vDC to redis-1/redis-2, but this is no longer a concern in AWS.
Update Rummager configuration so it uses backend-redis, and remove api-redis.
Everything relying on a single redis instance could potentially have impact if there is an issue with that Elasticache instance.
We will save money on not running multiple Elasticache instances.