Bump express from 4.19.2 to 4.21.0 in /demo

Bumps [express](https://github.com/expressjs/express) from 4.19.2 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@4.19.2...4.21.0)

---
updated-dependencies:
- dependency-name: express
  dependency-type: indirect
...

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