Bump js-yaml from 3.14.0 to 4.2.0 in /site-ui

Bumps [js-yaml](https://github.com/nodeca/js-yaml) from 3.14.0 to 4.2.0.
- [Changelog](https://github.com/nodeca/js-yaml/blob/master/CHANGELOG.md)
- [Commits](nodeca/js-yaml@3.14.0...4.2.0)

---
updated-dependencies:
- dependency-name: js-yaml
  dependency-version: 4.2.0
  dependency-type: direct:development
...

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

Author: dependabot[bot]