Update dependency postcss to v8.5.15 (#607)

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

Author: renovate[bot]

package-lock.json

@@ -42,7 +42,7 @@
     "eslint": "9.39.4",
     "eslint-config-stylelint": "26.1.0",
     "npm-run-all2": "8.0.4",
-    "postcss": "8.5.14",
+    "postcss": "8.5.15",
     "prettier": "3.8.3",
     "remark-cli": "12.0.1",
     "stylelint": "16.26.1"