Use Sidekiq.dump_json for serializing arguments

This will serialize the arguments into JSON, just like Sidekiq would
for calculating the `#idempotency_string` and checking for duplicate
jobs.

This would be a more robust approach, because serialization of for
example symbols would currently be different when serialized on the
clientside versus after they have been loaded from redis.

It would not have affected anything on a running instance, because the
key is already serialized into the job hash and checked like that in
Sidekiq server.

lib/gitlab/sidekiq_middleware/duplicate_jobs/duplicate_job.rb

@@ -133,11 +133,7 @@ module Gitlab
         end
 
         def idempotency_string
-          # TODO: dump the argument's JSON using `Sidekiq.dump_json` instead
-          # this should be done in the next release so all jobs are written
-          # with their idempotency key.
-          # see https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/1090
-          "#{worker_class_name}:#{arguments.join('-')}"
+          "#{worker_class_name}:#{Sidekiq.dump_json(arguments)}"
         end
       end
     end

spec/lib/gitlab/sidekiq_middleware/duplicate_jobs/duplicate_job_spec.rb

@@ -13,7 +13,7 @@ RSpec.describe Gitlab::SidekiqMiddleware::DuplicateJobs::DuplicateJob, :clean_gi
   let(:queue) { 'authorized_projects' }
 
   let(:idempotency_key) do
-    hash = Digest::SHA256.hexdigest("#{job['class']}:#{job['args'].join('-')}")
+    hash = Digest::SHA256.hexdigest("#{job['class']}:#{Sidekiq.dump_json(job['args'])}")
     "#{Gitlab::Redis::Queues::SIDEKIQ_NAMESPACE}:duplicate:#{queue}:#{hash}"
   end