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

The ci_stages table is one of the top 9 tables at risk of a Primary Key Overflow risk.

With #327622 (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 new Primary Key
• Concurrently add a Unique index for the PK (ci_stages_pkey)
• There are no additional indexes that include the converted column (id), so new indexes will be required.
• Single Transaction: Drop old PK, define new PK, swap sequence, swap columns

Previous Implementation Issue: #327622 (closed)