fix(v2): remove no longer used postcss-present-env from dependencies

Author: armano2

packages/docusaurus/package.json

@@ -91,7 +91,6 @@
     "pnp-webpack-plugin": "^1.6.4",
     "postcss": "^8.2.7",
     "postcss-loader": "^4.1.0",
-    "postcss-preset-env": "^6.7.0",
     "prompts": "^2.4.0",
     "react-dev-utils": "^11.0.1",
     "react-helmet": "^6.1.0",

website/docs/lifecycle-apis.md

@@ -361,14 +361,7 @@ By default, `postcssOptions` looks like this:
 ```js
 const postcssOptions = {
   ident: 'postcss',
-  plugins: [
-    require('postcss-preset-env')({
-      autoprefixer: {
-        flexbox: 'no-2009',
-      },
-      stage: 4,
-    }),
-  ],
+  plugins: [require('autoprefixer')],
 };
 ```