Convert ci_job_artifacts.id to bigint - Step 2: Finalize background migrations and swap columns

The ci_job_artifacts table is one of the largest tables in GitLab's database still uses an integer (int4) Primary Key. It has a high Primary Key Overflow risk as can be seen on its tracking issue.

With #325614 (closed) we have started the process of converting it to bigint (int8) type.

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

• Clean up the background migrations

• Drop the DEFAULT 0 from the affected columns

• Concurrently add a Unique index for the PK (ci_job_artifacts_pkey)

• Concurrently add any additional indexes that include the converted columns

  ci_job_artifacts:
    "expired_artifacts_temp_index" btree (id, created_at) WHERE expire_at IS NULL AND date(timezone('UTC'::text, created_at)) < '2020-06-22'::date
    "index_ci_job_artifacts_for_terraform_reports" btree (project_id, id) WHERE file_type = 18
    "index_ci_job_artifacts_id_for_terraform_reports" btree (id) WHERE file_type = 18
  
    "index_ci_job_artifacts_on_job_id_and_file_type" UNIQUE, btree (job_id, file_type)
    "index_ci_job_artifacts_on_expire_at_and_job_id" btree (expire_at, job_id)
  
  project_pages_metadata:
    "index_project_pages_metadata_on_artifacts_archive_id" btree (artifacts_archive_id)

• Single Transaction: Drop old PK, define new PK, swap sequence, swap columns

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

• Drop the old columns

Previous Implementation Issue: #325614 (closed)

Related Tracking Issue: #276019 (closed)