Update pipeline to run specs against a Redis Cluster

What does this MR do and why?

This MR adds .use-redis-cluster to the pipeline to run Redis-Cluster compatible components against a Redis Cluster in CI. This is because Gitlab::Redis::RateLimiting runs against a Redis Cluster in gstg and gprd. Gitlab::Redis::Cache is up next as part of gitlab-com/gl-infra&878 (closed). Rate-limiting operations are fairly safe but cache will be prone to being non-compatible with Redis Cluster due to multi-key operations which may not be cross-slot safe.

We need the CI pipeline to run against an actual Redis Cluster to catch such failures before it gets deployed in the wild.

Some notable parts of this MR

• It uses an image from https://hub.docker.com/r/grokzen/redis-cluster -- the plan is to build our own redis-cluster image in https://gitlab.com/gitlab-org/gitlab-build-images. i'm using an off-the-shelf image to get started
• It adds redis-only jobs to pg13's unit, system and integration rspecs/rspec-ee -- avoid blowing up the number of specs if we test for every permutation
• The rediscluster service runs a Redis Cluster of 3 master nodes and 0 replica nodes. It exposes 3 ports (7001, 7002, 7003).

Refer to gitlab-com/gl-infra/scalability#2327 (closed)

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How to set up and validate locally

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MR acceptance checklist

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