Limit concurrency of security update schedulers

What does this MR do and why?

We don't want to have multiple schedulers run unbounded and exhaust all Sidekiq resources, and spin up way too many pipelines as well. This will make it a lot safer to enable this at scale, and allows admins to configure it to their needs. Not every instance can handle the default, and not every instance needs to limit it to that level.

References

Relates to #600052

Screenshots or screen recordings

Before	After

How to set up and validate locally

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