Remove grunt-autoprefixer in favor of grunt-postcss with autoprefixer

grunt-autoprefixer was deprecated - https://github.com/nDmitry/grunt-autoprefixer/commit/e020f878d5fe1a3f9f15bc7533b31c12ffa74799
None of the autoprefixer tasks should have been called directly, so they are now removed.  `grunt precommit` and `grunt build` still work exactly as they should.  This change doesn't affect the output of our builds.

An NPM install is required after you have updated after this change.

See #31700
Props netweb



git-svn-id: https://develop.svn.wordpress.org/trunk@33129 602fd350-edb4-49c9-b593-d223f7449a82

package.json

@@ -9,8 +9,8 @@
   "author": "The WordPress Contributors",
   "license": "GPL-2.0+",
   "devDependencies": {
+    "autoprefixer-core": "~5.2.1",
     "grunt": "~0.4.5",
-    "grunt-autoprefixer": "~3.0.0",
     "grunt-browserify": "~3.8.0",
     "grunt-contrib-clean": "~0.6.0",
     "grunt-contrib-compress": "~0.13.0",
@@ -26,6 +26,7 @@
     "grunt-jsvalidate": "~0.2.2",
     "grunt-legacy-util": "^0.2.0",
     "grunt-patch-wordpress": "~0.3.0",
+    "grunt-postcss": "~0.5.4",
     "grunt-rtlcss": "~1.6.0",
     "grunt-sass": "~1.0.0",
     "matchdep": "~0.3.0"