Bump serve-static and express in /backend

Bumps [serve-static](https://github.com/expressjs/serve-static) to 1.16.2 and updates ancestor dependency [express](https://github.com/expressjs/express). These dependencies need to be updated together.


Updates `serve-static` from 1.15.0 to 1.16.2
- [Release notes](https://github.com/expressjs/serve-static/releases)
- [Changelog](https://github.com/expressjs/serve-static/blob/v1.16.2/HISTORY.md)
- [Commits](expressjs/serve-static@v1.15.0...v1.16.2)

Updates `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: serve-static
  dependency-type: indirect
- dependency-name: express
  dependency-type: direct:production
...

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