Upgrade postcss-mixins to 12.1

Author: shepmaster

ui/frontend/package.json

@@ -62,7 +62,7 @@
     "normalize.css": "^8.0.0",
     "postcss": "^8.2.7",
     "postcss-loader": "^8.1.0",
-    "postcss-mixins": "^11.0.1",
+    "postcss-mixins": "^12.1.2",
     "postcss-nesting": "^13.0.0",
     "postcss-simple-vars": "^7.0.1",
     "prettier": "^3.0.3",