Twenty Twenty-One: Bring back PostCSS config.

Twenty Twenty-One uses PostCSS to convert the custom properties for Internet Explorer. Without a config file, the postcss command does nothing, causing issues in IE. This brings back the config from GitHub, which replaces all custom properties with the default values.

Props t-p, poena, ryelle.
Merges [49800] to the 5.6 branch.
Fixes #52040.
Built from https://develop.svn.wordpress.org/branches/5.6@49824


git-svn-id: http://core.svn.wordpress.org/branches/5.6@49543 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-content/themes/twentytwentyone/postcss.config.js

@@ -0,0 +1,12 @@
+module.exports = {
+	plugins: [
+		require('postcss-nested'),
+		require('postcss-css-variables')({
+			preserve: false,
+			preserveAtRulesOrder: true
+		}),
+		require('postcss-calc')({
+			precision: 0
+		})
+	]
+};

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.6.1-alpha-49821';
+$wp_version = '5.6.1-alpha-49824';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.