Convert `ci_pipeline_variables.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	!123132 (merged)	workflowcomplete	%16.2
Confirm backfill is finished and Async Create index	!125188 (merged) !128719 (merged)	workflowcomplete workflowcomplete	%16.3
Sync Create index	!127449 (merged)	workflowcomplete	%16.3
Add not valid foreign key constraint Async validate foreign key constraint	!127445 (merged)	workflowverification	%16.3
Sync validate foreign key constraint	!129199 (merged)	workflowin dev	%16.4
Swap the columns			%16.4
Remove the trigger and old integer columns			%16.5
Remove ignore rules			%16.6