fix: Use browserslist-config-cozy instead of custom autoprefixer config

In postcss config, we gave the browsers list to autoprefixer plugin
directly. But the `browsers` property is deprecated and will be removed
in next major version (see
https://github.com/postcss/autoprefixer/releases/tag/9.6.0).

We also use autoprefixer through autoprefixer-stylus, but it uses a very
old version of autoprefixer (7.1.0) that doesn't understand the `dead`
query that our browserslist config uses. A PR fixing this have been
merged and will hopefuly be released soon, see
jescalan/autoprefixer-stylus#180. That's why I
left the custom browserslist config for autoprefixer-stylus in
`build:css`, `build:css:utils` and `build:css:kss` scripts for now. But
we should upgrade this module and stop using a custom browserslist
config for it as soon as possible.

package.json

@@ -187,5 +187,8 @@
     "snapshotSerializers": [
       "enzyme-to-json/serializer"
     ]
-  }
+  },
+  "browserslist": [
+    "extends browserslist-config-cozy"
+  ]
 }

postcss.config.js

@@ -1,12 +1,9 @@
-const browsers = require('browserslist-config-cozy')
-
 module.exports = {
   plugins: [
     require('cssnano')({
       preset: [
         'advanced',
         {
-          autoprefixer: { browsers },
           mergeIdents: false,
           reduceIdents: false,
           zindex: false