Update postcss-loader dependency to ^4.1.0.

Author: ocean90

package-lock.json

@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+### Breaking Changes
+
+-   The bundled `postcss-loader` dependency has been updated from requiring `^3.0.0` to requiring `^4.1.0` ([#27821](https://github.com/WordPress/gutenberg/pull/27821)).
+
 ## 14.0.0 (2021-03-17)
 
 ### Breaking Changes

packages/scripts/CHANGELOG.md

@@ -64,7 +64,7 @@
 		"mini-css-extract-plugin": "^0.9.0",
 		"minimist": "^1.2.0",
 		"npm-package-json-lint": "^5.0.0",
-		"postcss-loader": "^3.0.0",
+		"postcss-loader": "^4.1.0",
 		"prettier": "npm:wp-prettier@2.2.1-beta-1",
 		"puppeteer-core": "^5.5.0",
 		"read-pkg-up": "^1.0.1",