Update dependency postcss to v8.5.15 (#1112)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

package.json

@@ -54,7 +54,7 @@
     "markdownlint-cli": "0.48.0",
     "next": "16.2.6",
     "npm-run-all2": "8.0.4",
-    "postcss": "8.5.14",
+    "postcss": "8.5.15",
     "prettier": "3.8.3",
     "react": "19.2.6",
     "react-dom": "19.2.6",