Move update_head_pipeline_for_merge_request queue to `pipeline_processing` namespace

What does this MR do?

• CE: Here
• EE: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4862

Currently, update_head_pipeline_for_merge_request is being processed under pipeline_default: 3 namespace.

This should be processed under pipeline_processing: 5, because it really impacts user experiences on gitlab.com.

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@ayufan said

Well. I would probably just move pipeline_default:update_head_pipeline_for_merge_request to pipeline_processing. And possibly introduce additional pipeline_background:create_trace_artifact and gave it weight of 1 in sidekiq_queues.yml.

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17314#note_61817030

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

closes https://gitlab.com/gitlab-org/gitlab-ce/issues/43919