Update dependency postcss-loader to v4.3.0 (#336)

Co-authored-by: Renovate Bot <bot@renovateapp.com>

Author: renovate[bot]

package-lock.json

@@ -82,7 +82,7 @@
     "postcss": "8.3.6",
     "postcss-color-function": "4.1.0",
     "postcss-import": "13.0.0",
-    "postcss-loader": "4.1.0",
+    "postcss-loader": "4.3.0",
     "postcss-nested": "4.2.3",
     "postcss-preset-env": "6.7.0",
     "prettier": "npm:wp-prettier@^2.0.4",