chore(deps): update cssnano and remove unused clean-css (sugarlabs#5256)

- cssnano: 6.0.1 → 6.1.2 (minor update)
- Remove clean-css: unused direct dependency (gulp-clean-css bundles its own)

Part of sugarlabs#4668

Author: vyagh

package-lock.json

@@ -25,7 +25,6 @@
     "@babel/core": "^7.28.5",
     "@babel/eslint-parser": "^7.28.5",
     "@babel/preset-env": "^7.28.5",
-    "clean-css": "^4.2.4",
     "cypress": "^14.0.2",
     "electron": "^39.2.7",
     "electron-builder": "^26.0.12",
@@ -48,7 +47,7 @@
     "@tonejs/midi": "^2.0.28",
     "autoprefixer": "^10.4.16",
     "compression": "^1.8.1",
-    "cssnano": "^6.0.1",
+    "cssnano": "^6.1.2",
     "express": "^4.21.2",
     "gulp-concat": "^2.6.1",
     "gulp-postcss": "^10.0.0",