Bump send and express in /backend

Bumps [send](https://github.com/pillarjs/send) to 0.19.0 and updates ancestor dependency [express](https://github.com/expressjs/express). These dependencies need to be updated together.


Updates `send` from 0.18.0 to 0.19.0
- [Release notes](https://github.com/pillarjs/send/releases)
- [Changelog](https://github.com/pillarjs/send/blob/master/HISTORY.md)
- [Commits](pillarjs/send@0.18.0...0.19.0)

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: send
  dependency-type: indirect
- dependency-name: express
  dependency-type: direct:production
...

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