change postcss to 8.x (#343)

Author: wardpeet

package.json

@@ -66,7 +66,7 @@
     "style-inject": "^0.3.0"
   },
   "peerDependencies": {
-    "postcss": "^8.1.14"
+    "postcss": "8.x"
   },
   "xo": {
     "extends": "rem",
@@ -103,4 +103,4 @@
       "next"
     ]
   }
-}
+}