Bump postcss from 8.4.19 to 8.4.31 (librenms#15380)

Bumps [postcss](https://github.com/postcss/postcss) from 8.4.19 to 8.4.31.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@8.4.19...8.4.31)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

package.json

@@ -19,7 +19,7 @@
         "laravel-mix": "^6.0.41",
         "lodash": "^4.17.21",
         "popper.js": "^1.16.1",
-        "postcss": "^8.4.5",
+        "postcss": "^8.4.31",
         "resolve-url-loader": "^5.0.0",
         "sass": "^1.49.0",
         "sass-loader": "^12.4.0",