Adjust the use of `Vulnerabilities::FindingsPreloader` which is dependent on feedback

Implementation plan:

Vulnerabilities::FindingsPreloader is used to efficiently preload the feedback of a given list of vulnerabilities/findings. We need to adjust this read operation after removal of Vulnerabilities::Feedback so that we do not depend on feedback model anymore when deprecate_vulnerabilities_feedback FF is on. This preloader is currently used in two places:

• Adjust the use of Vulnerabilities::FindingsPreloader
  • In ee/lib/api/vulnerability_findings.rb
  • In ee/app/serializers/vulnerabilities/finding_serializer.rb