chore(deps): update express requirement in /backend

Updates the requirements on [express](https://github.com/expressjs/express) to permit the latest version.
- [Release notes](https://github.com/expressjs/express/releases)
- [Changelog](https://github.com/expressjs/express/blob/4.21.2/History.md)
- [Commits](expressjs/express@4.21.1...4.21.2)

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

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

backend/package.json

@@ -7,7 +7,7 @@
     "dev": "nodemon src/index.js"
   },
   "dependencies": {
-    "express": "^4.21.1",
+    "express": "^4.21.2",
     "cors": "^2.8.5",
     "dockerode": "^4.0.2",
     "multer": "^1.4.5-lts.1",