Add an init method to prepare for Puma 7

What does this MR do and why?

With https://github.com/puma/puma/pull/3297, Puma 7 raises an exception if Puma's options are accessed before they are fully loaded. Currently we pass these options during loading of the Puma config file, which then triggers this exception. These options are used to determine whether Puma is in clustered mode by checking the number of workers.

To prepare for Puma 7 and avoid this exception, add a set_puma_worker_count method that registers the number of workers directly.

How to set up and validate locally

See the instructions in gitlab-development-kit!5249 (merged).

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