npm(deps): bump postcss in the core-dependencies group

Bumps the core-dependencies group with 1 update: [postcss](https://github.com/postcss/postcss).


Updates `postcss` from 8.5.13 to 8.5.15
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@8.5.13...8.5.15)

---
updated-dependencies:
- dependency-name: postcss
  dependency-version: 8.5.15
  dependency-type: direct:production
  update-type: version-update:semver-patch
  dependency-group: core-dependencies
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

apps/webpack-example/package.json

@@ -29,7 +29,7 @@
     "file-loader": "^6.2.0",
     "html-webpack-plugin": "^5.6.7",
     "mini-css-extract-plugin": "^2.10.0",
-    "postcss": "^8.5.13",
+    "postcss": "^8.5.15",
     "postcss-loader": "^8.2.1",
     "react-refresh": "^0.18.0",
     "serve": "^14.2.6",

packages/nextjs-plugin/package.json

@@ -53,7 +53,7 @@
     "@stylexswc/typescript-config": "0.16.3",
     "@types/node": "^25.6.0",
     "next": "^16.2.6",
-    "postcss": "^8.5.13",
+    "postcss": "^8.5.15",
     "react": "^19.2.5",
     "react-dom": "^19.2.5",
     "webpack": "^5.106.2"

packages/postcss-plugin/package.json

@@ -36,7 +36,7 @@
     "fast-glob": "^3.3.3",
     "glob-parent": "^6.0.2",
     "is-glob": "^4.0.3",
-    "postcss": "^8.5.13"
+    "postcss": "^8.5.15"
   },
   "devDependencies": {
     "@stylexswc/eslint-config": "0.16.3",