Set default concurrency limit based on urgency & max threads

What does this MR do and why?

Set a default concurrency_limit for all workers based on their urgency and maximum possible threads in the shard. The maximum threads is derived from max pods * concurrency of the shard.

The max pods is retrieved from a new SIDEKIQ_MAX_REPLICAS environment variable is added in gitlab-org/charts/gitlab!4348.

Proposed percentage based on urgency gitlab-com/gl-infra/data-access/durability/team#215 (comment 2571342290):

• High: 35%
• Low: 25%
• Throttled: 15%

References

gitlab-com/gl-infra/data-access/durability/team#215 (closed)

Also as 2nd prerequisite of gitlab-com/gl-infra/observability/team#3815 (closed)

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How to set up and validate locally

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