chore(deps-dev): bump cssnano from 5.0.3 to 5.0.15

Bumps [cssnano](https://github.com/cssnano/cssnano) from 5.0.3 to 5.0.15.
- [Release notes](https://github.com/cssnano/cssnano/releases)
- [Commits](https://github.com/cssnano/cssnano/compare/[email protected]@5.0.15)

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

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