Build(deps): Bump prismjs from 1.17.1 to 1.21.0 in /examples/express (#…

…1657)

Bumps [prismjs](https://github.com/PrismJS/prism) from 1.17.1 to 1.21.0.
- [Release notes](https://github.com/PrismJS/prism/releases)
- [Changelog](https://github.com/PrismJS/prism/blob/master/CHANGELOG.md)
- [Commits](PrismJS/prism@v1.17.1...v1.21.0)

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>