Resolve "Set `vulnerability_export_parts` schema to`gitlab_sec`"

What does this MR do and why?

This MR sets the gitlab_schema for the vulnerability_export_parts table to gitlab_sec and makes it's organization_id reference into a foreign key as part of it's decomposition to the upcoming Sec DB. No other code performs joins to this table, so it's a simple modification.

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Related to #471793 (closed)