sandbox-npm(deps): bump express from 4.20.0 to 4.21.0 in /sandbox

Bumps [express](https://github.com/expressjs/express) from 4.20.0 to 4.21.0.
- [Release notes](https://github.com/expressjs/express/releases)
- [Changelog](https://github.com/expressjs/express/blob/4.21.0/History.md)
- [Commits](expressjs/express@4.20.0...4.21.0)

---
updated-dependencies:
- dependency-name: express
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

sandbox/package.json

@@ -9,7 +9,7 @@
   "license": "Unlicense",
   "private": true,
   "dependencies": {
-    "express": "^4.20.0",
+    "express": "^4.21.0",
     "loglevel": "^1.9.1",
     "uuid": "^10.0.0"
   },