Resolve "Set `vulnerability_finding_evidences` schema to`gitlab_sec`"

What does this MR do and why?

This MR sets the gitlab_schema for vulnerability_finding_evidences 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, or the decomposition FAQ relating to the overall effort and the effects some of these changes will or will not have on the MR.

MR acceptance checklist

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Related to #471796 (closed)