Replace `cssnext`  with `postcss-preset-env`


### `Details`

Replace `cssnext` with `postcss-preset-env` in README examples, since `cssnext` is deprecated with `postcss-preset-env` as it's direct successor

### `Error (Logs|Stacks)`

- None

### `Reproduction [Code]`

- Not needed

### `Environment`

- Not needed