Bump helmet from 7.2.0 to 8.1.0 in /backend

Bumps [helmet](https://github.com/helmetjs/helmet) from 7.2.0 to 8.1.0.
- [Changelog](https://github.com/helmetjs/helmet/blob/main/CHANGELOG.md)
- [Commits](https://github.com/helmetjs/helmet/compare/v7.2.0...v8.1.0)

---
updated-dependencies:
- dependency-name: helmet
  dependency-version: 8.1.0
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

backend/package.json

@@ -21,7 +21,7 @@
     "dotenv": "^17.4.2",
     "express": "^4.18.2",
     "express-async-errors": "^3.1.1",
-    "helmet": "^7.1.0",
+    "helmet": "^8.1.0",
     "ioredis": "^5.3.2",
     "jsonwebtoken": "^9.0.2",
     "morgan": "^1.10.0",