Remove references to project_vulnerability_feedback_index_path

Before we can remove the deprecated controller Projects::VulnerabilityFeedbackController we need to remove all references to it's routes from the code base.

Remove references from ee/app/helpers/ee/projects_helper.rb

The following helpers in ee/app/helpers/ee/projects_helper.rb refernce URLs to the Projects::VulnerabilityFeedbackController:

• create_vulnerability_feedback_issue_path
• create_vulnerability_feedback_merge_request_path
• create_vulnerability_feedback_dismissal_path

The presence or absence of the REST URLs are used to determine if a user can create an issue from a finding or dismiss a vulnerability. (https://gitlab.com/gitlab-org/gitlab/-/blob/1984adf2f46442dcb83cf212a2e5895bf0349d69/ee/app/assets/javascripts/security_dashboard/components/pipeline/security_dashboard_table_row.vue#L64-75 and https://gitlab.com/gitlab-org/gitlab/-/blob/1984adf2f46442dcb83cf212a2e5895bf0349d69/ee/app/assets/javascripts/security_dashboard/components/pipeline/security_dashboard_vuex.vue#L65-74)

These can be replaced using push_frontend_ability.

Remove references from ee/app/serializers/ee/merge_request_widget_entity.rb

Used in EE::MergeRequestWidgetEntity to set vulnerability_feedback_path.

Only seems to to be referenced in ee/app/assets/javascripts/vue_merge_request_widget/stores/mr_widget_store.js to set this.vulnerabilityFeedbackPath. That variable seems to be orphaned, so I think this whole reference chain can just be removed.

Remove references from ee/app/serializers/vulnerabilities/feedback_entity.rb

Used for Vulnerabilities::FeedbackEntity#destroy_vulnerability_feedback_dismissal_path. This property seems to be unused and can be removed.