Fix to use self and downstreams builds for duration calculation

What does this MR do and why?

This MR is to use self and downstreams builds duration instead of self and bridge job duration for upstream pipeline duration.

This issue #387942 (closed) found out that some of the downstream pipelines took a long time (e.g. 6 hours) to kick off running and for example finished in 20mins, which means the duration for a bridge job becomes 6 hours and 20 mins. And user expects to see 20mins.

Screenshots or screen recordings

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How to set up and validate locally

1. Enable the feature flag:

   rails runner 'Feature.enable(:ci_use_downstream_pipeline_duration_for_calculation)'

2. Create a project 356759-test-duration
3. Create .gitlab-ci.yml with the following content:

   # .gitlab-ci.yml
   run-child-pipeline:
     trigger:
       include: ci/child-pipeline.yml
       strategy: depend
   
   # ci/child-pipeline.yml
   stages:
     - build
     - test
   
   child-job:
     stage: build
     script:
       - echo "Running child job..."
       - sleep 20
       - echo "Running complete."
   
   run-grand-child-pipeline:
     stage: test
     trigger:
       include: ci/grand-child-pipeline.yml
       strategy: depend
   
   # ci/grand-child-pipeline.yml
   grand-child-job:
     script:
       - echo "Running grand child job..."
       - sleep 15
       - echo "Running complete."

4. Check if the pipeline takes 35 seconds

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #356759 (closed)