Bump mongodb from 2.6.0 to 2.8.2

Bumps [mongodb](https://github.com/mongodb/mongo-rust-driver) from 2.6.0 to 2.8.2.
- [Release notes](https://github.com/mongodb/mongo-rust-driver/releases)
- [Commits](mongodb/mongo-rust-driver@v2.6.0...v2.8.2)

---
updated-dependencies:
- dependency-name: mongodb
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Author: dependabot[bot]