Disable cssnano's colormin plugin (#15347)

It produces odd rgba values which also seem to cause issues in monaco's
color parser where the scoll shadow went red for some reason.

Regression by: https://github.com/go-gitea/gitea/pull/15333

webpack.config.js

@@ -93,6 +93,7 @@ export default {
               discardComments: {
                 removeAll: true,
               },
+              colormin: false,
             },
           ],
         },