Bump mongoose from 7.8.6 to 8.13.1 in /backend

Bumps [mongoose](https://github.com/Automattic/mongoose) from 7.8.6 to 8.13.1.
- [Release notes](https://github.com/Automattic/mongoose/releases)
- [Changelog](https://github.com/Automattic/mongoose/blob/master/CHANGELOG.md)
- [Commits](Automattic/mongoose@7.8.6...8.13.1)

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

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

Author: dependabot[bot]

Diff for: backend/package-lock.json

@@ -53,7 +53,7 @@
     "lodash-es": "^4.17.21",
     "mjml": "5.0.0-alpha.6",
     "mongodb": "^6.14.0",
-    "mongoose": "7.8.6",
+    "mongoose": "8.13.1",
     "mongoose-delete": "^1.0.2",
     "morgan": "^1.10.0",
     "nanoid": "^5.1.5",