css: update browserslist requirement.

Update the minimum browser requirement from browsers released since
2012, to browsers released in the last 5 years.

This means PostCSS won't prefix CSS properties like `flex` or other
properties supported by browsers since 2017. Babel also won't transpile
Javascript syntax that has been supported by browsers since 2017.

package.json

@@ -60,7 +60,6 @@
     ]
   },
   "browserslist": [
-    "defaults",
-    "since 2012"
+    "last 5 years"
   ]
 }