Good bye 2.4, welcome 3.x! (#3732)

As decided on #3731

UPGRADE-2.4.md

@@ -1,4 +1,4 @@
-UPGRADE FROM 2.3 to 2.4
+UPGRADE FROM 2.x to 3.0
 =======================
 
 ### Dependencies
@@ -33,4 +33,4 @@ If you're using a custom implementation of `sonata.admin.route_loader` service,
 
 ## LegacyModelsToArrayTransformer
 
-The ``ModelsToArrayTransformer`` has been renamed to ``LegacyModelsToArrayTransformer``. ``ModelsToArrayTransformer`` should be only be used with SF2.7+
+The ``ModelsToArrayTransformer`` has been renamed to ``LegacyModelsToArrayTransformer``. ``ModelsToArrayTransformer`` should be only be used with SF2.7+

composer.json

@@ -62,7 +62,7 @@
     },
     "extra": {
         "branch-alias": {
-            "dev-master": "2.4.x-dev"
+            "dev-master": "3.x-dev"
         }
     }
 }