DAST on-demand site profiles: prevent error banner from showing

This commit puts a feature flag before a request to prevent an
error-banner from showing due to accessing a non-existing graphql
endpoint.

ee/app/assets/javascripts/dast_site_profiles_form/components/dast_site_profile_form.vue

@@ -152,8 +152,11 @@ export default {
   async created() {
     if (this.isEdit) {
       this.validateTargetUrl();
+
+      if (this.glFeatures.securityOnDemandScansSiteValidation) {
         await this.fetchValidationStatus();
       }
+    }
   },
   methods: {
     validateTargetUrl() {