build(deps): bump postcss-merge-longhand from 5.1.7 to 6.0.0

Bumps [postcss-merge-longhand](https://github.com/cssnano/cssnano) from 5.1.7 to 6.0.0.
- [Release notes](https://github.com/cssnano/cssnano/releases)
- [Commits](https://github.com/cssnano/cssnano/compare/postcss-merge-longhand@5.1.7...postcss-merge-longhand@6.0.0)

---
updated-dependencies:
- dependency-name: postcss-merge-longhand
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

package.json

@@ -55,7 +55,7 @@
     "ora": "^5.1.0",
     "postcss": "^8.4.21",
     "postcss-import": "^15.0.0",
-    "postcss-merge-longhand": "^5.1.7",
+    "postcss-merge-longhand": "^6.0.0",
     "posthtml": "^0.16.6",
     "posthtml-attrs-parser": "^0.1.1",
     "posthtml-base-url": "^2.0.0",