Bump prismjs from 1.20.0 to 1.21.0
Bump prismjs from 1.20.0 to 1.21.0

Bumps [prismjs](https://github.com/PrismJS/prism) from 1.20.0 to 1.21.0.
- [Release notes](https://github.com/PrismJS/prism/releases)
- [Changelog](https://github.com/PrismJS/prism/blob/master/CHANGELOG.md)
- [Commits](https://github.com/PrismJS/prism/compare/v1.20.0...v1.21.0)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>
Polymer/lit-html,lit/lit,Polymer/lit-html,lit/lit-element-starter-js,lit/lit,Polymer/lit-html,Polymer/lit-html,PolymerLabs/lit-element-starter-js,lit/lit-element-starter-js,lit/lit,PolymerLabs/lit-element-starter-js,lit/lit