Bump cssnano from 5.1.5 to 5.1.7 (#901)

Bumps [cssnano](https://github.com/cssnano/cssnano) from 5.1.5 to 5.1.7.
- [Release notes](https://github.com/cssnano/cssnano/releases)
- [Commits](https://github.com/cssnano/cssnano/compare/[email protected]@5.1.7)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]