Build/Test Tools: Update PostCSS to version 8.

This updates PostCSS related dependencies for the main Core build process to versions that use the latest major version of PostCSS, 8.x.

There were no changes to any versioned CSS files as a result of this update directly. However, two occurrences of `-o-tab-size` have been removed due to the version of `caniuse-lite` used being updated in the process.

See #57554, #57555.

git-svn-id: https://develop.svn.wordpress.org/trunk@55150 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/css/common.css

@@ -3160,7 +3160,6 @@ img {
 	font-family: Consolas, Monaco, monospace;
 	font-size: 13px;
 	background: #f6f7f7;
-	-o-tab-size: 4;
 	tab-size: 4;
 }
 

src/wp-admin/css/customize-controls.css

@@ -1604,7 +1604,6 @@ p.customize-section-description {
 	font-family: Consolas, Monaco, monospace;
 	font-size: 12px;
 	padding: 6px 8px;
-	-o-tab-size: 2;
 	tab-size: 2;
 }
 .customize-control-code_editor textarea,