Sync validate foreign key for pipeline_id_convert_to_bigint

What does this MR do and why?

As titled, this is the second step to validate the foreign key synchronously for ci_pipeline_variables.pipeline_id.

See https://docs.gitlab.com/ee/development/database/add_foreign_key_to_existing_column.html#add-a-migration-to-validate-the-fk-synchronously

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Before	After

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 #422804 (closed)