Bump mongoose from 5.6.4 to 5.7.5 in /server

Bumps [mongoose](https://github.com/Automattic/mongoose) from 5.6.4 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@5.6.4...5.7.5)

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