Bump postcss-custom-properties from 12.1.11 to 13.3.4

Bumps [postcss-custom-properties](https://github.com/csstools/postcss-plugins/tree/HEAD/plugins/postcss-custom-properties) from 12.1.11 to 13.3.4.
- [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-major
...

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

package.json

@@ -75,7 +75,7 @@
     "postcss": "^8.4.2",
     "postcss-calc": "^9.0.1",
     "postcss-custom-media": "^9.0.1",
-    "postcss-custom-properties": "12.1.11",
+    "postcss-custom-properties": "13.3.4",
     "postcss-import": "^15.0.1",
     "postcss-loader": "^7.0.2",
     "postcss-media-minmax": "^5.0.0",