title: PostCSS Plugin errors with Rails 6 and TailwindCSS
body: <p>Have a new rails 6.0.3.4 application which was using TailwindCSS version 1.9.  Having just attempted to upgrade to the latest TailwindCSS release I'm getting the following errors:</p>
  <pre><code>ERROR in ./app/javascript/stylesheets/application.scss (./node_modules/css-loader/dist/cjs.js??ref--7-1!./node_modules/postcss-loader/src??ref--7-2!./node_modules/sass-loader/dist/cjs.js??ref--7-3!./app/javascript/stylesheets/application.scss)
  
  Module build failed (from ./node_modules/postcss-loader/src/index.js):
  
  Error: PostCSS plugin tailwindcss requires PostCSS 8.
  
  Migration guide for end-users:
  https://github.com/postcss/postcss/wiki/PostCSS-8-for-end-users
  </code></pre>
  <p>This error persisted when attempting to run</p>
  <blockquote>
  <p>bin/webpack</p>
  </blockquote>
  <p>Couldn't find any information on how to upgrade to PostCSS version 8.</p>