3.5: MongoDb driver deprecation warning

## To Reproduce

Start the application. The warning `isConnected is deprecated and will be removed in the next major version` is printed out in the console output.

## Expected behavior
No deprecation warning.

## Describe the bug
From the [documentation of the currently used driver](https://mongodb.github.io/node-mongodb-native/3.6/reference/unified-topology/#mongoclient-connect-isconnected), isConnected method is deprecated and will be removed in the next major version.

If `--trace-warnings` is used when starting the app, it's seen the problem is caused by the direct apostrophe dependency `connect-mongo@3.2.0` module used by Apostrophe. The `isConnected` use is removed in v4 of the module.

The migration to v4 of the module seems painless according to their [migration guide](https://github.com/jdesboeufs/connect-mongo/blob/master/MIGRATION_V4.md)

## Details

**Version of Node.js:**
v12.21.0

**Apostrophe:**
apostrophe@3.5.0 (against `main` branch)

**MongoDB driver:**
mongodb@3.7.2 (as reported by `npm ls`)

Start by reproducing the warning with --trace-warnings and inspect the direct connect-mongo@3.2.0 dependency identified in the report. Review the linked connect-mongo v4 migration guide and verify that starting the application no longer emits the isConnected deprecation warning.

Written by the indexing model from the issue text.