Add concurrency limit to Advanced Search

What does this MR do and why?

We've just merged Add Concurrency limit Sidekiq Middleware (!139851 - merged)

This MR adds an application setting to ElasticCommitIndexerWorker, which uses the concurrency limit middleware.

Screenshots or screen recordings

How to set up and validate locally

1. In rails console enable the experiment fully

   Feature.enable(:sidekiq_concurrency_limit_middleware)

2. Visit Advanced Search settings http://localhost:3000/admin/application_settings/advanced_search#js-elasticsearch-settings
3. Update the Code indexing concurrency field

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #435402 (closed)