core: fix db pagination filter

packages/hydrooj/src/service/db.ts

@@ -144,12 +144,8 @@ export class MongoService extends Service {
         cursor: FindCursor<T>, page: number, pageSize: number,
     ): Promise<[docs: T[], numPages: number, count: number]> {
         if (page <= 0) throw new ValidationError('page');
-        let filter = {};
-        for (const key of Object.getOwnPropertySymbols(cursor)) {
-            if (key.toString() !== 'Symbol(filter)') continue;
-            filter = cursor[key];
-            break;
-        }
+        // this is for mongodb driver v6
+        const filter = (cursor as any).cursorFilter;
         const coll = this.db.collection(cursor.namespace.collection as any);
         const [count, pageDocs] = await Promise.all([
             Object.keys(filter).length ? coll.count(filter) : coll.countDocuments(filter),