Bump mongoose from 8.7.0 to 8.9.6 in /backend

Bumps [mongoose](https://github.com/Automattic/mongoose) from 8.7.0 to 8.9.6.
- [Release notes](https://github.com/Automattic/mongoose/releases)
- [Changelog](https://github.com/Automattic/mongoose/blob/master/CHANGELOG.md)
- [Commits](Automattic/mongoose@8.7.0...8.9.6)

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

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

Author: dependabot[bot]