Bump mongoose from 6.0.9 to 6.13.8 in /react-express-mongodb/backend

Bumps [mongoose](https://github.com/Automattic/mongoose) from 6.0.9 to 6.13.8.
- [Release notes](https://github.com/Automattic/mongoose/releases)
- [Changelog](https://github.com/Automattic/mongoose/blob/master/CHANGELOG.md)
- [Commits](https://github.com/Automattic/mongoose/compare/6.0.9...6.13.8)

---
updated-dependencies:
- dependency-name: mongoose
  dependency-type: indirect
...

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