Fix compatibility with MongoDB 4.4

Author: GromNaN

src/Schema/Builder.php

@@ -7,7 +7,6 @@
 use Closure;
 use MongoDB\Model\CollectionInfo;
 use MongoDB\Model\IndexInfo;
-use stdClass;
 
 use function array_keys;
 use function assert;
@@ -160,7 +159,7 @@ public function getColumns($table)
             [
                 '$group' => [
                     '_id' => '$fields.k',
-                    'total' => ['$count' => new stdClass()],
+                    'total' => ['$sum' => 1],
                     'types' => ['$addToSet' => ['$type' => '$fields.v']],
                 ],
             ],