Bump jsonwebtoken from 8.1.0 to 9.0.0 in /backend

Bumps [jsonwebtoken](https://github.com/auth0/node-jsonwebtoken) from 8.1.0 to 9.0.0.
- [Release notes](https://github.com/auth0/node-jsonwebtoken/releases)
- [Changelog](https://github.com/auth0/node-jsonwebtoken/blob/master/CHANGELOG.md)
- [Commits](auth0/node-jsonwebtoken@v8.1.0...v9.0.0)

---
updated-dependencies:
- dependency-name: jsonwebtoken
  dependency-type: direct:production
...

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Author: dependabot[bot]

backend/package-lock.json

@@ -14,7 +14,7 @@
     "express-jwt": "^5.3.0",
     "express-server-utils": "^1.1.0",
     "jade": "~1.11.0",
-    "jsonwebtoken": "^8.1.0",
+    "jsonwebtoken": "^9.0.0",
     "mongoose": "^4.13.0",
     "morgan": "~1.7.0",
     "passport": "^0.4.0",