Resolve "Set `vulnerability_finding_links` schema to`gitlab_sec`"

What does this MR do and why?

This MR sets the gitlab_schema for vulnerability_finding_links to gitlab_sec, changes the project_id foreign key to a loose foreign key as part of it's decomposition to the upcoming Sec Database, see parent epic for more context

MR acceptance checklist

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How to set up and validate locally

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

Related to #471797 (closed)