Adjust MergeRequestController not to use `Vulnerabilities::Feedback`

The MergeRequests Controller provides information to the merge request security report diff widget for pre-merge comparisons. This information provides links to findings which have had issues and merge requests created associated to them by providing the linking vulnerability feedback.

As we are deprecating the vulnerability feedback model, this needs to be appropriately flagged off.

Implementation Plan

• backend Implement serialisers for Vulnerability::IssueLink, Vulnerability::MergeRequestLink and Vulnerability::StateTransition
• backend Using the :deprecate_vulnerability_feedback FF, provide issue, merge request and dismissal information for diffs via the new serialisers rather than the Vulnerability::Feedback model in https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/serializers/vulnerabilities/feedback_entity.rb