Initialize the conversion for ci_sources_pipelines

What does this MR do and why?

Prepare to convert source_pipeline_id and pipeline_id to bigint for table ci_sources_pipelines, 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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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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• I have evaluated the MR acceptance checklist for this MR.

Related to #412869 (closed)