chore(deps): update dependency postcss-loader to v8

Author: renovate[bot]

gui/package-lock.json

@@ -32,7 +32,7 @@
     "lit-analyzer": "2.0.0-pre.2",
     "make-dir-cli": "3.0.0",
     "mini-css-extract-plugin": "2.6.1",
-    "postcss-loader": "7.0.1",
+    "postcss-loader": "8.0.0",
     "prettier": "2.7.1",
     "style-loader": "3.3.1",
     "terser-webpack-plugin": "5.3.3",