Fix TypeError post Stylelint update

`postcss-scss` expects `postcss@8`, this therefore caused the error `TypeError: Class extends value undefined is not a constructor or null` when running Stylelint

see: stylelint-scss/stylelint-config-standard-scss#5 (comment)

package.json

@@ -39,6 +39,7 @@
     "jquery-xpath": "^0.3.1",
     "jquery.iframe-transport": "^1.0.0",
     "jsrender": "^1.0.11",
+    "postcss": "^8.4.12",
     "stickyfilljs": "^2.1.0",
     "webpack-concat-plugin": "^3.0.0"
   },