[FIX] web: correctly target 'o_dropdown_item' child

Partially revert b0c325 removing rules defining '<a>' children as
exceptions. Indeed, to have a child wrapping the entry's content (either
<a> or <span>) is now considered the norm.

This commit will extend bootstrap 'dropdown-item' design to any
'o_dropdown_item' child rather than on 'o_dropdown_item' itself.

Part-of: odoo/odoo#73311

addons/web/static/src/core/dropdown/dropdown.scss

@@ -122,6 +122,14 @@
     }
   }
 
+  &_item > *:first-child:last-child {
+    @extend .dropdown-item;
+  }
+
+  &_item.o_dropdown_active > *:first-child:last-child {
+    @extend .dropdown-item:hover;
+  }
+
   &_menu_group {
     @extend .dropdown-header;
   }