Update dependency postcss to v8.4.18 (#2074)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

package-lock.json

@@ -85,7 +85,7 @@
     "mini-css-extract-plugin": "1.6.2",
     "npm-run-all": "4.1.5",
     "pleasantest": "2.2.0",
-    "postcss": "8.4.17",
+    "postcss": "8.4.18",
     "postcss-inline-svg": "4.1.0",
     "postcss-loader": "4.3.0",
     "postcss-media-minmax": "5.0.0",