Bump cssnano from 5.0.17 to 5.1.15

Bumps [cssnano](https://github.com/cssnano/cssnano) from 5.0.17 to 5.1.15.
- [Release notes](https://github.com/cssnano/cssnano/releases)
- [Commits](https://github.com/cssnano/cssnano/compare/cssnano@5.0.17...cssnano@5.1.15

)

---
updated-dependencies:
- dependency-name: cssnano
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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