Bump send, express and serve-static in /slides

Bumps [send](https://github.com/pillarjs/send) to 0.19.0 and updates ancestor dependencies [send](https://github.com/pillarjs/send), [express](https://github.com/expressjs/express) and [serve-static](https://github.com/expressjs/serve-static). 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 4.20.0 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)

Updates `serve-static` from 1.16.0 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@1.16.0...v1.16.2)

---
updated-dependencies:
- dependency-name: send
  dependency-type: indirect
- dependency-name: express
  dependency-type: direct:development
- dependency-name: serve-static
  dependency-type: indirect
...

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

Author: dependabot[bot]