Display username field of ServiceNow as textbox (#178002)

## Summary

Display the `Username` field of native ServiceNow connector as textbox,
instead of textarea.

### For maintainers

- [ ] This was checked for breaking API changes and was [labeled
appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

packages/kbn-search-connectors/types/native_connectors.ts

@@ -3542,7 +3542,7 @@ export const NATIVE_CONNECTOR_DEFINITIONS: Record<string, NativeConnector | unde
       username: {
         default_value: null,
         depends_on: [],
-        display: DisplayType.TEXTAREA,
+        display: DisplayType.TEXTBOX,
         label: i18n.translate('searchConnectors.nativeConnectors.servicenow.username.label', {
           defaultMessage: 'Username',
         }),