Explore increasing throughput of load simulation on Staging

Part of Simulate load on Staging (&7320 - closed) Emulation Goal:Stage#3.

Depends on Increase load simulation coverage on Staging (#351230 - closed) and related to analysis on https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/14948#note_819118753

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Steps:

• Explore supporting multiple users to bypass limit if concurrency is increased
• Explore increasing concurrency on Staging (gstg)
  • web - #351231 (comment 1028184838)
  • api - #351231 (comment 1028184838)
  • git - #351231 (comment 1027783140)
  • registry - #351231 (comment 1031127590)
  • pages - #351231 (comment 1031130941) blocked by https://gitlab.com/gitlab-com/gl-infra/cmbr/-/issues/9
• Verify that increased throughput doesn't affect QA pipelines
• Increase concurrency once majority of Staging traffic is migrated to gstg-cny (depends on gitlab-com/gl-infra&608 (closed))