Update dependency postcss to v8.4.13

Author: renovate-bot

package-lock.json

@@ -22,7 +22,7 @@
         "eslint-loader": "^4.0.2",
         "eslint-plugin-import": "^2.22.1",
         "eslint-plugin-vue": "^7.20.0",
-        "postcss": "^8.2.8",
+        "postcss": "8.4.13",
         "resolve-url-loader": "^3.1.4",
         "stylelint": "^13.13.1",
         "stylelint-config-standard": "^21.0.0",