Use Gitlab::Redis::RepositoryCache Redis instance

What does this MR do and why?

As part of gitlab-com/gl-infra/scalability#2077 (closed):

• To use the Redis instance Gitlab::Redis::RepositoryCache added in !107187 (merged) with MultiStore to let the data migrated naturally.
• To use MultiStore feature flag as the switch for traffic to go to repository cache instance.

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How to set up and validate locally

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MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.