Pin postcss to 8.5.23 via npm override

Co-authored-by: abdulahmad307 <204748719+abdulahmad307@users.noreply.github.com>

Author: Copilot

package-lock.json

@@ -29,6 +29,9 @@
     "markdownlint-cli2": "^0.23.1",
     "vitest": "^4.1.9"
   },
+  "overrides": {
+    "postcss": "8.5.23"
+  },
   "repository": {
     "type": "git",
     "url": "git+https://github.com/github/markdownlint-github.git"