Bump yaml and cssnano in /htdocs

Removes [yaml](https://github.com/eemeli/yaml). It's no longer used after updating ancestor dependency [cssnano](https://github.com/cssnano/cssnano). These dependencies need to be updated together.


Removes `yaml`

Updates `cssnano` from 5.1.12 to 6.0.0
- [Release notes](https://github.com/cssnano/cssnano/releases)
- [Commits](https://github.com/cssnano/cssnano/compare/[email protected]@6.0.0)

---
updated-dependencies:
- dependency-name: yaml
  dependency-type: indirect
- dependency-name: cssnano
  dependency-type: direct:development
...

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

Author: dependabot[bot]