chore(deps): update dependency postcss to v8.5.15

Author: renovate[bot]

package.json

@@ -101,7 +101,7 @@
         "gulp-uglify": "3.0.2",
         "gulp-zip": "5.1.0",
         "ordered-read-streams": "2.0.0",
-        "postcss": "8.5.14",
+        "postcss": "8.5.15",
         "postcss-easy-import": "4.0.0",
         "pump": "3.0.4"
     }