Change defer_sidekiq_jobs FF delimiter to _

What does this MR do and why?

Change defer_sidekiq_jobs FF delimiter to _

The delimiter of : does not work well in Slack because it could be converted to emoji, e.g. :Gitlab::xxx or :Chaos::xxx. This makes managing the feature flag via ChatOps unusable.

Changelog: changed

Check this comment for more context: gitlab-com/gl-infra/scalability#2347 (comment 1399297375)

---

This feature flag was introduced in !120606 (merged) to allow us to defer sidekiq jobs during an incident (without manually running rake tasks / console commands).

How to set up and validate locally

1. Check out this branch and restart Sidekiq process

2. In Rails console, enable the feature flag Feature.enable(:"defer_sidekiq_jobs_Chaos::SleepWorker")

3. Check that the feature flag is enabled

   [14] pry(main)> Feature.enabled?(:"defer_sidekiq_jobs_Chaos::SleepWorker", type: :worker, default_enabled_if_undefined: false)
   => true

4. Clear the ScheduledSet jobs Sidekiq::ScheduledSet.new.clear

5. Run the worker Chaos::SleepWorker.perform_async(1)

6. Check that the job is now in the ScheduledSet

   [26] pry(main)> Sidekiq::ScheduledSet.new.entries
   => [#<Sidekiq::SortedEntry:0x000000012b711e58
     @args=nil,
     @item=
      {"retry"=>3,
       "queue"=>"default",
       "backtrace"=>true,
       "version"=>0,
       "queue_namespace"=>"chaos",
       "class"=>"Chaos::SleepWorker",
       "args"=>[1],
       "jid"=>"fbaa1109d7d85fbe4c56d991",
       "created_at"=>1684751642.898051,
       "correlation_id"=>"112a79778a43675422559409ba15ce83",
       "meta.caller_id"=>"Chaos::SleepWorker",
       "meta.feature_category"=>"not_owned",
       "meta.root_caller_id"=>"Chaos::SleepWorker",
       "worker_data_consistency"=>"always",
       "size_limiter"=>"validated",
       "scheduled_at"=>1684751942.8980029},
     @parent=#<Sidekiq::ScheduledSet:0x000000012b7183e8 @_size=1, @name="schedule">,
     @queue="default",
     @score=1684751942.8980029,
     @value=
      "{\"retry\":3,\"queue\":\"default\",\"backtrace\":true,\"version\":0,\"queue_namespace\":\"chaos\",\"class\":\"Chaos::SleepWorker\",\"args\":[1],\"jid\":\"fbaa1109d7d85fbe4c56d991\",\"created_at\":1684751642.898051,\"correlation_id\":\"112a79778a43675422559409ba15ce83\",\"meta.caller_id\":\"Chaos::SleepWorker\",\"meta.feature_category\":\"not_owned\",\"meta.root_caller_id\":\"Chaos::SleepWorker\",\"worker_data_consistency\":\"always\",\"size_limiter\":\"validated\",\"scheduled_at\":1684751942.8980029}">]

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