Replace vulnerabilities_feedback REST call with GraphQL mutation in MR security widget modal

At the time of writing the Create Issue button on the Merge Request Security tab widget popup for a finding calls the Projects::VulnerabilityFeedbackController#create action.

We intend to remove this controller in Remove Projects::VulnerabilityFeedbackController (#419214) so we need to update the Create Issue button to not call this controller.

Implementation Outline

1. Within ee/app/assets/javascripts/vue_shared/security_reports/components/modal.vue change createNewIssue method to use GraphQL mutation instead of POST request to REST endpoint
2. Update specs