Bump mongoose from 5.6.9 to 5.7.5 in /backend

Bumps [mongoose](https://github.com/Automattic/mongoose) from 5.6.9 to 5.7.5.
- [Release notes](https://github.com/Automattic/mongoose/releases)
- [Changelog](https://github.com/Automattic/mongoose/blob/master/History.md)
- [Commits](Automattic/mongoose@5.6.9...5.7.5)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]