fix(deps): update dependency postcss to v8.4.21 (#11130)

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

install/package.json

@@ -110,7 +110,7 @@
         "passport-local": "1.0.0",
         "pg": "8.8.0",
         "pg-cursor": "2.7.4",
-        "postcss": "8.4.20",
+        "postcss": "8.4.21",
         "postcss-clean": "1.2.0",
         "progress-webpack-plugin": "1.0.16",
         "prompt": "1.3.0",