Add the concurrency_limit for the Zoekt::DeleteProjectWorker

What does this MR do and why?

Adding the concurrency limit to the Search::Zoekt::DeleteProjectWorker to reduce the load on the Zoekt webserver. We found that during reallocation, there were a lot of simultaneous delete requests to the Zoekt web server, due to which the Zoekt server was giving a Bad Gateway error. It is safe to enable the FF zoekt_delete_project_worker_concurrency by default.

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Related to #471647