Merge branch '6.3' into 6.4

* 6.3:
  remove duplicated allow-plugins section

Author: xabbuh

composer.json

@@ -61,10 +61,7 @@
         "preferred-install": {
             "*": "dist"
         },
-        "sort-packages": true,
-        "allow-plugins": {
-            "symfony/flex": true
-        }
+        "sort-packages": true
     },
     "autoload": {
         "psr-4": {