Tweak select form fields class (#3559)

Bootstrap 5 uses `form-select` (https://getbootstrap.com/docs/5.0/forms/select/) to help indicate the field is a dropdown.

Author: codealchemy

src/rails_admin/filter-box.js

@@ -21,7 +21,7 @@ import flatpickr from "flatpickr";
 
       if (operators.length > 0) {
         control = $(
-          '<select class="form-control form-control-sm"></select>'
+          '<select class="form-control form-select form-select-sm"></select>'
         ).prop("name", operator_name);
 
         operators.forEach((operator) => {