Spin up a new Redis cluster for repository cache data

Based on the data from #1819 (closed) we can consider the size of the Redis instances we want to add. Likely we'll want to have this instance in Kubernetes. If we do, that would live in tanka, where we can add a new cluster in environments/redis/main.jsonnet.

We might have to make more options on the allocated resources configurable for doing so.

We should be able to determine the allocated memory and maxmemory using the repository_cache_write_bytes metric from #1819 (closed). The current TTL for these keys is 2 weeks in the worst case. Coming from the default configuration of the Rails.cache. But we should aim for being able to keep the values for ~1 day. When the caches are separated, we can consider lowering the TTL for everything stored in this instance without affecting everything in Rails.cache.