Switch RedisCacheable to use Redis cache instance
What does this MR do?
This used the 'shared state' Redis due to an understandable misunderstanding about the purpose of that. The shared state Redis is for persistent data, and the cache Redis is for caching short-lived data.
Despite the names, both the cache and the shared state instance are available to all ways the Rails application is deployed (web nodes and background job nodes in particular).
This will lead to a short period of reading uncached data, but that should be OK as this is simply a cache. See gitlab-foss!16756 (comment 327305091) for more details.