Add support for pymongo 4.x

Relates to https://github.com/closeio/closeio/issues/34125

To officially support MongoDB 6.0+ we need to upgrade to pymongo 4+.

The upstream mongoengine added support for Pymongo 4 in [version (0.24+)](https://github.com/MongoEngine/mongoengine/blob/master/docs/changelog.rst#changes-in-0240). The [PR](https://github.com/MongoEngine/mongoengine/pull/2614).