Convert ci_sources_pipelines.source_job_id to bigint - Step 2: Finalize background migrations and swap columns

With #328435 (closed) we have started the process of converting the foreign key from ci_sources_pipelines that references ci_builds (ci_sources_pipelines.source_job_id) to bigint (int8).

In this issue we'll address the second step of the process:

• Clean up the background migrations

• Concurrently add any secondary indexes that include the source_job_id column

  "index_ci_sources_pipelines_on_source_job_id" btree (source_job_id)

• Single Transaction: Drop old FK, define new FK (as invalid), swap columns, validate FK