Bump express from 4.16.4 to 4.21.0 in /frontend

Bumps [express](https://github.com/expressjs/express) from 4.16.4 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@4.16.4...4.21.0)

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

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