Replace `.exists` with `EXISTS ()`

`.exists` should not be used because it's an internal ActiveRecord
method, but we can easily generate the same sql query with `EXISTS`.

app/services/boards/issues/list_service.rb

@@ -63,7 +63,7 @@ module Boards
       def without_board_labels(issues)
         return issues unless board_label_ids.any?
 
-        issues.where.not(issues_label_links.limit(1).arel.exists)
+        issues.where.not('EXISTS (?)', issues_label_links.limit(1))
       end
 
       def issues_label_links
@@ -71,10 +71,8 @@ module Boards
       end
 
       def with_list_label(issues)
-        issues.where(
-          LabelLink.where("label_links.target_type = 'Issue' AND label_links.target_id = issues.id")
-                   .where("label_links.label_id = ?", list.label_id).limit(1).arel.exists
-        )
+        issues.where('EXISTS (?)', LabelLink.where("label_links.target_type = 'Issue' AND label_links.target_id = issues.id")
+                                            .where("label_links.label_id = ?", list.label_id).limit(1))
       end
     end
   end