Add comments warning against application code use in Vulnerability Feedback background migrations

!107367 (merged) and !97699 (merged) Both leverage application code to accomplish there migrations, but this contravenes the GitLab best practice to inline code for background migrations as this may lead to the migrations executing in undesired fashions by late adopting self-managed users if later GitLab changes modify these dependencies. We should prevent other developers from using this as a standard accepted practice by adding comments warning against it.

Implementation Plan

• [] Add comments in !107367 (merged) and !97699 (merged) where application code is being referenced (in the background migration and in the spec) stating it's discouraged to prevent its use as an example