Bump mongoose from 8.0.3 to 8.4.0 in /backend

Bumps [mongoose](https://github.com/Automattic/mongoose) from 8.0.3 to 8.4.0.
- [Release notes](https://github.com/Automattic/mongoose/releases)
- [Changelog](https://github.com/Automattic/mongoose/blob/master/CHANGELOG.md)
- [Commits](Automattic/mongoose@8.0.3...8.4.0)

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

Signed-off-by: dependabot[bot] <[email protected]>