Initialize the migration to convert pk for ci_pipeline_variables

What does this MR do and why?

Prepare to convert pk to bigint for table ci_pipeline_variables, see the first step https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#migrating-integer-primary-keys-to-bigint

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How to set up and validate locally

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

MR acceptance checklist

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Related to #408936 (closed)