Bump clean-css from 5.1.3 to 5.1.5

Bumps [clean-css](https://github.com/jakubpawlowicz/clean-css) from 5.1.3 to 5.1.5.
- [Release notes](https://github.com/jakubpawlowicz/clean-css/releases)
- [Changelog](https://github.com/clean-css/clean-css/blob/master/History.md)
- [Commits](clean-css/clean-css@v5.1.3...v5.1.5)

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

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

Author: dependabot[bot]

package-lock.json

@@ -41,7 +41,7 @@
     "@11ty/eleventy": "^0.12.1",
     "@11ty/eleventy-plugin-syntaxhighlight": "^3.1.2",
     "all-contributors-cli": "^6.20.0",
-    "clean-css": "^5.1.3",
+    "clean-css": "^5.1.5",
     "html-minifier": "^4.0.0",
     "luxon": "^2.0.2",
     "markdown-it": "^12.1.0",