Replace deprecated button on new epic creation form

changelogs/unreleased/new-epic-form-deprecated-button.yml

+---
+title: Replace deprecated button on new epic creation form
+merge_request: 57874
+author:
+type: changed

ee/app/assets/javascripts/epic/components/epic_form.vue

@@ -227,7 +227,7 @@ export default {
       <div class="footer-block row-content-block gl-display-flex">
         <gl-button
           type="submit"
-          variant="success"
+          variant="confirm"
           :loading="loading"
           :disabled="!title"
           data-testid="save-epic"