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

The ci_builds_metadata 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 #325616 (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_builds_metadata_pkey)

• Concurrently add any additional indexes that include the converted columns

    "index_ci_builds_metadata_on_build_id" UNIQUE, btree (build_id)
    "index_ci_builds_metadata_on_build_id_and_has_exposed_artifacts" btree (build_id) WHERE has_exposed_artifacts IS TRUE
    "index_ci_builds_metadata_on_build_id_and_id_and_interruptible" btree (build_id) INCLUDE (id) WHERE interruptible = true

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

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

• Drop the old columns

Previous Implementation Issue: #325616 (closed)

Related Tracking Issue: #276020 (closed)