Backend: Fix N+1 in previousStageJobs

Summary

An N+1 was discovered in previousStageJobs when the work in #422079 (comment 1536884321) was being discussed.

Proposal

The N+1 should be fixed before #389195 (closed) is implemented.

Additional details

Some relevant technical details, if applicable, such as:

• Does this need a feature flag?
• Does there need to be an associated instrumentation issue created related to this work?
• Is there an example response showing the data structure that should be returned (new endpoints only)?
• What permissions should be used?
• Is this EE or CE?
  • EE
  • CE
• Additional comments:

Implementation Table

Group	Issue Link
backend	👈 You are here
backend	Backend/Frontend: Add new `previousStagesJobs` ... (#389195 - closed)
backend	Backend: Fix N+1 in previousStageJobsAndNeeds (#422079 - closed)
backend	Backend: Remove GraphQL field - previousStageJo... (#424417)

Links/References