chore(deps): update dependency autoprefixer to v10.4.21

Author: renovate[bot]

package-lock.json

@@ -23,7 +23,7 @@
   },
   "devDependencies": {
     "@tryghost/release-utils": "0.8.1",
-    "autoprefixer": "10.4.19",
+    "autoprefixer": "10.4.21",
     "beeper": "2.1.0",
     "cssnano": "5.1.15",
     "gscan": "4.43.1",