Fix z index in stat trial form modal (#26798)

webapp/channels/src/components/start_trial_form_modal/start_trial_form_modal.scss

@@ -111,13 +111,12 @@
         .DropdownInput {
             margin-top: 36px;
 
+
             .DropDown__menu {
-                z-index: 999;
                 max-height: 124px;
             }
 
             .DropDown__menu-list {
-                z-index: 999;
                 max-height: 124px;
             }
         }
@@ -136,7 +135,6 @@
         }
 
         .countries-section {
-            z-index: 999;
             max-height: 164px;
         }