Update to PostCSS 8

[XhmikosR]

Just opening an issue to track this :)

[hudochenkov]

Blocked by stylelint stylelint/stylelint#4942. Plugin could not be using PostCSS 8 until stylelint using it.

[machinshin]

stylelint has migrated over to PostCSS8.

  "extends": "stylelint-config-sass-guidelines",

now fails

postcss-sorting: postcss.plugin was deprecated. Migration guide:
https://evilmartians.com/chronicles/postcss-8-plugin-migration
TypeError: Cannot read properties of undefined (reading 'unprefixed')
    at /foo/app/assets/stylesheets/_svg.scss:5:5
    at processEveryNode (/foo/node_modules/stylelint-order/rules/properties-alphabetical-order/checkNode.js:26:43)
    at Rule.each (/foo/node_modules/postcss/lib/container.js:41:16)
    at checkNode (/foo/node_modules/stylelint-order/rules/properties-alphabetical-order/checkNode.js:11:7)
    at processRulesAndAtrules (/foo/node_modules/stylelint-order/rules/properties-alphabetical-order/index.js:55:5)
    at /foo/node_modules/postcss/lib/container.js:55:18
    at Rule.each (/foo/node_modules/postcss/lib/container.js:41:16)
    at Rule.walk (/foo/node_modules/postcss/lib/container.js:52:17)
    at /foo/node_modules/postcss/lib/container.js:60:24
    at AtRule.each (/foo/node_modules/postcss/lib/container.js:41:16)
    at AtRule.walk (/foo/node_modules/postcss/lib/container.js:52:17)

[hudochenkov]

@machinshin Use stylelint-order@5.0.0.

[hudochenkov]

@XhmikosR it is now using PostCSS 8 in version 5.