Update default Sidekiq max_concurrency doc

What does this MR do and why?

Previously, the default was changed in !104989 (merged) from 50 to 20, but some places are not yet updated like the bin/sidekiq-cluster binary help text.

How to set up and validate locally

On master:

master ?2 ❯ bin/sidekiq-cluster --help
cli.rb [QUEUE,QUEUE] [QUEUE] ... [OPTIONS]

Options:
    -h, --help                       Shows this help message
    -m, --max-concurrency INT        Maximum threads to use with Sidekiq (default: 50, 0 to disable)
        --min-concurrency INT        Minimum threads to use with Sidekiq (default: 0)
    -e, --environment ENV            The application environment
    -P, --pidfile PATH               Path to the PID file
    -r, --require PATH               Location of the Rails application
        --queue-selector             Run workers based on the provided selector
    -n, --negate                     Run workers for all queues in sidekiq_queues.yml except the given ones
    -i, --interval INT               The number of seconds to wait between worker checks
    -t, --timeout INT                Graceful timeout for all running processes
    -d, --dryrun                     Print commands that would be run without this flag, and quit
        --list-queues                List matching queues, and quit

On this branch:

mg-update-si…default-docs ?2 ❯ bin/sidekiq-cluster --help
cli.rb [QUEUE,QUEUE] [QUEUE] ... [OPTIONS]

Options:
    -h, --help                       Shows this help message
    -m, --max-concurrency INT        Maximum threads to use with Sidekiq (default: 20, 0 to disable)
        --min-concurrency INT        Minimum threads to use with Sidekiq (default: 0)
    -e, --environment ENV            The application environment
    -P, --pidfile PATH               Path to the PID file
    -r, --require PATH               Location of the Rails application
        --queue-selector             Run workers based on the provided selector
    -n, --negate                     Run workers for all queues in sidekiq_queues.yml except the given ones
    -i, --interval INT               The number of seconds to wait between worker checks
    -t, --timeout INT                Graceful timeout for all running processes
    -d, --dryrun                     Print commands that would be run without this flag, and quit
        --list-queues                List matching queues, and quit

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