Storybook dep postcss-loader needs to be upgraded.

[glomotion]

Describe the bug
The dependency version of postcss-loader that is installed with storybook is too old to use newer plugins like: https://github.com/postcss/postcss-mixins

To Reproduce

1. Init a basic storybook
2. add the preset to ./storybook/main.js storybook-css-modules-preset
3. set postcss.config.js to include the plugin: postcss-mixins
4. do npm run build-storybook
5. See error:
   ERR! Module build failed (from ./node_modules/@storybook/core/node_modules/postcss-loader/src/index.js): ERR! Error: PostCSS plugin postcss-mixins requires PostCSS 8. ERR! Migration guide for end-users: ERR! https://github.com/postcss/postcss/wiki/PostCSS-8-for-end-users

Code snippets

module.exports = {
  stories: ['../src/**/*.stories.mdx', '../src/**/*.stories.@(js|jsx|ts|tsx)'],
  addons: [
    '@storybook/addon-links',
    '@storybook/addon-essentials',
    '@storybook/addon-knobs',
    'storybook-css-modules-preset',
  ],
};

System

Environment Info:

  System:
    OS: macOS 10.15.7
    CPU: (8) x64 Intel(R) Core(TM) i7-3615QM CPU @ 2.30GHz
  Binaries:
    Node: 14.14.0 - ~/.nvm/versions/node/v14.14.0/bin/node
    Yarn: 1.22.4 - /usr/local/bin/yarn
    npm: 6.14.8 - ~/.nvm/versions/node/v14.14.0/bin/npm
  Browsers:
    Chrome: 86.0.4240.183
    Edge: 86.0.622.63
    Firefox: 80.0.1
    Safari: 14.0
  npmPackages:
    @storybook/addon-actions: ^6.0.28 => 6.0.28 
    @storybook/addon-essentials: ^6.0.28 => 6.0.28 
    @storybook/addon-knobs: ^6.0.28 => 6.0.28 
    @storybook/addon-links: ^6.0.28 => 6.0.28 
    @storybook/react: ^6.0.28 => 6.0.28 

[shilman]

closing as dupe to #12668