Merge pull request #670 from textpattern/dependabot/npm_and_yarn/autoprefixer-10.4.21

Bump autoprefixer from 10.4.20 to 10.4.21

Author: philwareham

package.json

@@ -11,7 +11,7 @@
   },
   "devDependencies": {
     "@lodder/grunt-postcss": "3.1.1",
-    "autoprefixer": "10.4.20",
+    "autoprefixer": "10.4.21",
     "cssnano": "7.0.6",
     "grunt": "1.6.1",
     "grunt-cli": "1.5.0",