Preload downstream pipelines when serializing bridge jobs

What does this MR do and why?

Completes the rollout of preload_ci_bridge_downstream_pipelines. This preload makes pipeline stages load faster for stages with many bridge jobs triggering downstream pipelines.

The flag has been enabled on GitLab.com for over 24h now without issues.

See !142199 (merged) for details.

Rollout issue gitlab-com/gl-infra/production#17473

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