Bump express from 4.17.1 to 4.21.2 in /examples/contexts

Bumps [express](https://github.com/expressjs/express) from 4.17.1 to 4.21.2.
- [Release notes](https://github.com/expressjs/express/releases)
- [Changelog](https://github.com/expressjs/express/blob/4.21.2/History.md)
- [Commits](expressjs/express@4.17.1...4.21.2)

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

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]