♻️ Use mongodb promises

[`mongodb@5`][1] removes support for using callbacks, forcing consumers
to use promises instead.

This non-breaking change does the minimum amount of work to move us to
using `mongodb` promises, while keeping all our interfaces intact.

[1]: https://github.com/mongodb/node-mongodb-native/releases/tag/v5.0.0

Author: alecgibson