Bump mongoose from 6.12.6 to 8.1.2 in /backend

Bumps [mongoose](https://github.com/Automattic/mongoose) from 6.12.6 to 8.1.2.
- [Release notes](https://github.com/Automattic/mongoose/releases)
- [Changelog](https://github.com/Automattic/mongoose/blob/master/CHANGELOG.md)
- [Commits](Automattic/mongoose@6.12.6...8.1.2)

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

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

backend/package.json

@@ -11,7 +11,7 @@
         "update"    : "npx npm-check-updates"
     },
     "dependencies"    : {
-        "mongoose" : "^6.7.2",
+        "mongoose" : "^8.1.2",
         "uuid"     : "^9.0.0"
     },
     "devDependencies" : {