Convert `ci_stages.pipeline_id` to bigint

Problem

This column references the ci_pipelines table and its column type is not bigint.

Process

Following the process outlined here, we must:

Step	MR	Workflow Status	Milestone
Initialize the conversion and ignore column	!126650 (merged)	workflowverification	%16.3
Confirm backfill is finished and Async Create index	!127471 (merged)	workflowin dev	%16.4
Sync Create index and add foreign key constraint			%16.4
Swap the columns			%16.4
Remove the trigger and old integer columns			%16.5
Remove ignore rules			%16.6