Make newPivotQuery function public

PR https://github.com/laravel/framework/pull/31677 made newPivotQuery function public and that's the reason why builds are failing.

src/Jenssegers/Mongodb/Relations/BelongsToMany.php

@@ -265,7 +265,7 @@ class BelongsToMany extends EloquentBelongsToMany
     /**
      * @inheritdoc
      */
-    protected function newPivotQuery()
+    public function newPivotQuery()
     {
         return $this->newRelatedQuery();
     }