Migrate button variants

Changelog: changed
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/74718
EE: true

ee/app/assets/javascripts/related_items_tree/components/create_epic_form.vue

@@ -194,7 +194,7 @@ export default {
       <gl-button
         :disabled="isSubmitButtonDisabled"
         :loading="isSubmitting"
-        variant="success"
+        variant="confirm"
         category="primary"
         type="submit"
         class="float-left"

ee/app/assets/javascripts/related_items_tree/components/create_issue_form.vue

@@ -245,7 +245,7 @@ export default {
       <div class="col-sm flex-sm-grow-0 mb-2 mb-sm-0">
         <gl-button
           class="w-100"
-          variant="success"
+          variant="confirm"
           category="primary"
           :disabled="!selectedProject || itemCreateInProgress"
           :loading="itemCreateInProgress || recentItemFetchInProgress"