Revert "Merge branch '218393-deprecate-sourced-pipelines' into 'master'"

What does this MR do and why?

As per #384053 (comment 1190976254), the MR Remove `has_many: sourced_pipelines` associatio... (!103980 - merged) caused 500 errors when customers use a single job to trigger multiple downstream pipelines via API using CI_JOB_TOKEN.

For now we need to revert the MR and evaluate the need for a more graceful deprecation in %16.0.

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