Add sidekiq_remove_jobs migration for TrendingProjectsWorker

See #553408 (closed) for context

Step 2 of removing TrendingProjectsWorker. Adds a migration that clears any remaining enqueued instances from Sidekiq and removes the cron job entry from the scheduled worker set.

Reference: https://docs.gitlab.com/development/sidekiq/compatibility_across_updates/#removing-worker-classes

Prerequisite: #555342 (closed) must be merged and deployed.

Implementation Steps

1. Generate a regular (non-post-deployment) migration:

   rails g migration RemoveTrendingProjectsWorkerJobInstances

2. Implement the migration:

   class RemoveTrendingProjectsWorkerJobInstances < Gitlab::Database::Migration[2.1]
     disable_ddl_transaction!
   
     DEPRECATED_JOB_CLASSES = %w[TrendingProjectsWorker]
   
     def up
       Gitlab::SidekiqSharding::Validator.allow_unrouted_sidekiq_calls do
         job_to_remove = Sidekiq::Cron::Job.find('trending_projects_worker')
         job_to_remove.destroy if job_to_remove
       end
   
       sidekiq_remove_jobs(job_klasses: DEPRECATED_JOB_CLASSES)
     end
   
     def down
       # This migration removes enqueued job instances and cannot be undone.
     end
   end