Convert ci_builds_runner_session.build_id to bigint - Step 2: Finalize background migrations and swap columns

With #328444 (closed) we have started the process of converting the foreign key from ci_builds_runner_session that references ci_builds (ci_builds_runner_session.build_id) to bigint (int8).

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

• Clean up the background migrations

• Drop the DEFAULT 0 from the build_id_convert_to_bigint column

• Concurrently add any secondary indexes that include the build_id column

  "index_ci_builds_runner_session_on_build_id" UNIQUE, btree (build_id)

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