Upgrade to css-minimize-webpack-plugin 2.0

This upgrades cssnano to version 5, which fixes a bunch of bugs and uses
postcss 8 (avoiding to install multiple versions of postcss).

Author: stof

lib/plugins/optimize-css-assets.js

@@ -18,9 +18,7 @@ const applyOptionsCallback = require('../utils/apply-options-callback');
  * @return {object}
  */
 module.exports = function(webpackConfig) {
-    const minimizerPluginOptions = {
-        sourceMap: webpackConfig.useSourceMaps
-    };
+    const minimizerPluginOptions = {};
 
     return new CssMinimizerPlugin(
         applyOptionsCallback(webpackConfig.cssMinimizerPluginOptionsCallback, minimizerPluginOptions)

package.json

@@ -33,7 +33,7 @@
     "chalk": "^4.0.0",
     "clean-webpack-plugin": "^3.0.0",
     "css-loader": "^5.0.1",
-    "css-minimizer-webpack-plugin": "^1.1.5",
+    "css-minimizer-webpack-plugin": "^2.0.0",
     "fast-levenshtein": "^3.0.0",
     "friendly-errors-webpack-plugin": "^2.0.0-beta.1",
     "loader-utils": "^2.0.0",