MongoServerError on single replica set after upgrading to v9.6

### Prerequisites - [x] I have written a descriptive issue title - [x] I have searched existing issues to ensure the issue has not already been raised ### Issue - Node.js version: v24.15 - Mongoose version: 9.6/9.6.1 (Upgraded from 9.5) - MongoDB Version: 6.0 (Single Replica Set) - My test environment URL: `mongodb://localhost/test?replicaSet=rs0` I encountered an issue after upgrading from v9.5 to v9.6/9.6.1. In my test environment, I am now seeing the following error: ``` MongoServerError: Only servers in a sharded cluster can start a new transaction at the active transaction number ``` I am running this against a single replica set (rs0). This setup worked perfectly on v9.5, but the error started appearing immediately after the version bump. I am aware that MongoDB 6.0 is an older version, but it is currently the environment I need to support. It seems the new driver version might be initiating transactions or retryable writes in a way that MongoDB 6.0 interprets as a sharded-cluster-only command. Does anyone have an idea why this behavior changed in v9.6? Are there any specific flags or configurations I should use to maintain compatibility with MongoDB 6.0 in a non-sharded setup?