Transition vulnerability feedback to gitlab_sec

What does this MR do and why?

Transition vulnerability feedback to gitlab_sec

It was initially desired to completely remove the reliance on the vulnerability_feedback model ahead of the sec-decomposition as the model has been officially deprecated for a year. Unfortunately, the code was written to be reversible if there was an issue with the deprecation, meaning much was left in place that still relies on the model. To remove this reliance would require a substantial rewrite of both frontend and backend code.

This is very much a piece of technical debt we'd like to eliminate, but the scope of that removal is too large to justify the delay to the sec-decomposition that will be required to execute it. As such, we will unfortunately need to transition it instead.

Related to #471795 (closed)