Update dependency postcss-loader to v8

Author: renovate[bot]

Diff for: web/package.json

@@ -24,7 +24,7 @@
   "devDependencies": {
     "autoprefixer": "10.4.4",
     "postcss": "8.4.12",
-    "postcss-loader": "6.2.1",
+    "postcss-loader": "8.1.1",
     "tailwindcss": "3.0.23"
   }
 }