Update CSS preprocessors (#797)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

package.json

@@ -48,7 +48,7 @@
         "@tryghost/release-utils": "0.6.12",
         "autoprefixer": "10.2.5",
         "beeper": "2.1.0",
-        "cssnano": "4.1.10",
+        "cssnano": "5.1.12",
         "gscan": "4.22.0",
         "gulp": "4.0.2",
         "gulp-concat": "2.6.1",
@@ -59,7 +59,7 @@
         "inquirer": "8.0.0",
         "postcss": "8.2.8",
         "postcss-color-mod-function": "3.0.3",
-        "postcss-easy-import": "3.0.0",
+        "postcss-easy-import": "4.0.0",
         "pump": "3.0.0"
     },
     "browserslist": [