Swap the columns for ci_pipeline_variables

What does this MR do and why?

Swap the columns for ci_pipeline_variables.

This is the step to swap bigint conversion columns:

• pipeline_id
• pipeline_id_convert_to_bigint

See https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#swap-the-columns-release-n--1

The other steps before this one:

• Backfill check is done at !123132 (merged) and !128719 (merged)
• Index is created at !125188 (merged) and !127449 (merged)
• Foreign key is created at !127445 (merged) and !129199 (merged)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #422805 (closed)