Update dependency postcss-loader to v8 (#43516)

Co-authored-by: Renovate Bot <bot@renovateapp.com>

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/15117277044

Upstream-Ref: Automattic/jetpack@0e26e0a

Author: anomiex

package.json

@@ -88,7 +88,7 @@
 		"jest-environment-jsdom": "29.7.0",
 		"postcss": "8.4.47",
 		"postcss-custom-properties": "12.1.7",
-		"postcss-loader": "6.2.0",
+		"postcss-loader": "8.1.1",
 		"sass": "1.87.0",
 		"sass-loader": "16.0.5",
 		"size-limit": "11.1.6",