Update CSS preprocessors

Author: renovate[bot]

package.json

@@ -46,9 +46,9 @@
     "contributors": "https://github.com/TryGhost/Casper/graphs/contributors",
     "devDependencies": {
         "@tryghost/release-utils": "0.8.1",
-        "autoprefixer": "10.4.7",
+        "autoprefixer": "10.4.15",
         "beeper": "2.1.0",
-        "cssnano": "5.1.12",
+        "cssnano": "5.1.15",
         "gscan": "4.36.1",
         "gulp": "4.0.2",
         "gulp-concat": "2.6.1",
@@ -57,7 +57,7 @@
         "gulp-uglify": "3.0.2",
         "gulp-zip": "5.1.0",
         "inquirer": "8.2.4",
-        "postcss": "8.2.13",
+        "postcss": "8.4.28",
         "postcss-color-mod-function": "3.0.3",
         "postcss-easy-import": "4.0.0",
         "pump": "3.0.0"