fix(build): css stylesheets bundle config and deps (#6315)

package.json

@@ -121,6 +121,7 @@
     "cors": "^2.8.5",
     "cross-env": "=7.0.2",
     "css-loader": "^3.5.3",
+    "cssnano": "=4.1.10",
     "cypress": "=4.12.1",
     "dedent": "^0.7.0",
     "deepmerge": "^4.0.0",
@@ -153,8 +154,8 @@
     "npm-run-all": "^4.1.5",
     "oauth2-server": "^2.4.1",
     "open": "^7.0.4",
-    "optimize-css-assets-webpack-plugin": "^5.0.3",
     "postcss-loader": "^3.0.0",
+    "postcss-preset-env": "=6.7.0",
     "prettier": "^2.0.0",
     "raw-loader": "^4.0.0",
     "react-test-renderer": "^15.5.4",

webpack/stylesheets.babel.js

@@ -8,7 +8,6 @@
 import path from "path"
 import MiniCssExtractPlugin from "mini-css-extract-plugin"
 import IgnoreAssetsPlugin from "ignore-assets-webpack-plugin"
-import OptimizeCSSAssetsPlugin from "optimize-css-assets-webpack-plugin"
 
 export default {
   mode: "production",
@@ -33,7 +32,7 @@ export default {
             loader: "postcss-loader",
             options: {
               sourceMap: true,
-              plugins: loader => [
+              plugins: [
                 require("cssnano")(),
                 require("autoprefixer")(),
               ],