build(deps-dev): bump postcss-custom-properties from 13.3.4 to 13.3.6

Bumps [postcss-custom-properties](https://github.com/csstools/postcss-plugins/tree/HEAD/plugins/postcss-custom-properties) from 13.3.4 to 13.3.6.
- [Changelog](https://github.com/csstools/postcss-plugins/blob/main/plugins/postcss-custom-properties/CHANGELOG.md)
- [Commits](https://github.com/csstools/postcss-plugins/commits/HEAD/plugins/postcss-custom-properties)

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

Signed-off-by: dependabot[bot] <[email protected]>

package.json

@@ -105,7 +105,7 @@
     "postcss-advanced-variables": "3.0.1",
     "postcss-cli": "11.0.0",
     "postcss-color-function": "4.1.0",
-    "postcss-custom-properties": "13.3.4",
+    "postcss-custom-properties": "13.3.6",
     "postcss-jsx": "0.36.4",
     "postcss-load-config": "5.0.3",
     "postcss-syntax": "0.36.2",