build(deps): bump postcss-custom-properties from 12.1.11 to 13.3.12

Bumps [postcss-custom-properties](https://github.com/csstools/postcss-plugins/tree/HEAD/plugins/postcss-custom-properties) from 12.1.11 to 13.3.12.
- [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:production
  update-type: version-update:semver-major
...

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

package.json

@@ -45,7 +45,7 @@
     "css-loader": "^6.6.0",
     "http-proxy-middleware": "^2.0.6",
     "npm-check-updates": "^16.0.5",
-    "postcss-custom-properties": "^12.1.4",
+    "postcss-custom-properties": "^13.3.12",
     "react": "^16.14.0",
     "react-dom": "^16.14.0",
     "style-loader": "^3.3.1",