Bump mongoose from 6.2.6 to 6.4.6 in /backend

Bumps [mongoose](https://github.com/Automattic/mongoose) from 6.2.6 to 6.4.6.
- [Release notes](https://github.com/Automattic/mongoose/releases)
- [Changelog](https://github.com/Automattic/mongoose/blob/master/CHANGELOG.md)
- [Commits](Automattic/mongoose@6.2.6...6.4.6)

---
updated-dependencies:
- dependency-name: mongoose
  dependency-type: direct:production
...

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Author: dependabot[bot]