Update PostCSS

[omahane]

Description

Today, while restarting ddev, I noticed the terminal output repeated this line, which I'd never seen before:

remove-unwanted-comments-from-variables: postcss.plugin was deprecated. Migration guide:
https://evilmartians.com/chronicles/postcss-8-plugin-migration

Having visited the url in the message, it appears that there are steps to undertake to update PostCSS. Those steps are explained on that page.

Acceptance Criteria

• [ ] The deprecation message no longer appears in output when starting ddev

Team

Please check the team(s) that will do this work.

• [x] CMS Team
• [ ] Public Websites
• [ ] Facilities
• [ ] User support

[omahane]

I almost made this ticket again. I'm glad I looked for it.