Bump clean-css from 5.1.4 to 5.1.5

Bumps [clean-css](https://github.com/jakubpawlowicz/clean-css) from 5.1.4 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@v5.1.4...v5.1.5)

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

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

Author: dependabot[bot]

package-lock.json

@@ -26,7 +26,7 @@
     "@11ty/eleventy": "0.12.1",
     "bootstrap": "5.0.2",
     "bootstrap-icons": "1.5.0",
-    "clean-css": "^5.1.4",
+    "clean-css": "^5.1.5",
     "gulp": "^4.0.2",
     "gulp-clean": "^0.4.0",
     "gulp-clean-css": "^4.2.0",