Move `JobTracker` redis store to `SharedState`

What does this MR do and why?

In our continued effort to reduce Redis-Sidekiq load in preparation for Sidekiq Zonal Clusters, we want to move the LimitedCapacity::JobTracker logic to RedisSharedState. Unlike with our gradual transition of DuplicateJobs, we are doing a clean move here, as we estimate the impact of a temporary data duplication to be negligible. See this thread.

Part of gitlab-com/gl-infra/scalability#924 (closed)

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