Migrate confidential message to use PJ style class

Update the confidential warning message
in the "Create confidential merge request"
dropdown to use PJ compliant style classes.

app/assets/javascripts/confidential_merge_request/components/project_form_group.vue

@@ -87,7 +87,7 @@ export default {
     },
     showWarning() {
       if (this.warningText) {
-        this.warningText.classList.remove('hidden');
+        this.warningText.classList.remove('gl-display-none');
       }
 
       if (this.createBtn) {

app/views/projects/issues/_new_branch.html.haml

@@ -62,5 +62,5 @@
                 = create_mr_text
 
             - if can_create_confidential_merge_request?
-              %p.text-warning.js-exposed-info-warning.hidden
+              %p.gl-text-orange-500.js-exposed-info-warning.gl-display-none
                 = _('This may expose confidential information as the selected fork is in another namespace that can have other members.')