Update dependency postcss to v8

Author: renovate-bot

package.json

@@ -82,7 +82,7 @@
     "jest-cli": "26.4.2",
     "mkdirp": "1.0.4",
     "node-sass": "4.14.1",
-    "postcss": "7.0.34",
+    "postcss": "8.0.5",
     "postcss-combine-duplicated-selectors": "9.1.0",
     "postcss-custom-properties": "8.0.11",
     "postcss-remove-root": "0.0.2",