Update dependency c8 to v9 (postcss#469)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

package.json

@@ -33,7 +33,7 @@
   },
   "devDependencies": {
     "ava": "^3.1.0",
-    "c8": "^8.0.0",
+    "c8": "^9.0.0",
     "coveralls": "^3.0.0",
     "eslint": "^8.55.0",
     "eslint-config-problems": "8.0.0",