Fix prefer-object-spread in src/language-css (#7351)

src/language-css/parser-postcss.js

@@ -127,7 +127,7 @@ function flattenGroups(node) {
   }
 
   if (node.type === "paren_group" || node.type === "comma_group") {
-    return Object.assign({}, node, { groups: node.groups.map(flattenGroups) });
+    return { ...node, groups: node.groups.map(flattenGroups) };
   }
 
   return node;