Bump postcss from 8.5.5 to 8.5.6 in /src/Elastic.Documentation.Site (#1409)

Bumps [postcss](https://github.com/postcss/postcss) from 8.5.5 to 8.5.6.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@8.5.5...8.5.6)

---
updated-dependencies:
- dependency-name: postcss
  dependency-version: 8.5.6
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

src/Elastic.Documentation.Site/package-lock.json

@@ -40,7 +40,7 @@
     "globals": "16.2.0",
     "moment": "^2.30.1",
     "parcel": "2.15.2",
-    "postcss": "8.5.5",
+    "postcss": "8.5.6",
     "postcss-import": "16.1.0",
     "prettier": "3.5.3",
     "prettier-plugin-tailwindcss": "0.6.12",