Move website config to default branch

Adapts the website config to be compatible with the doctrine/doctrine-website#356 changes

Author: SenseException

.doctrine-project.json

@@ -4,16 +4,28 @@
   "slug": "doctrine-migrations-bundle",
   "versions": [
     {
-      "name": "2.2",
+      "name": "3.1",
       "branchName": "master",
-      "slug": "2.2",
+      "slug": "3.1",
       "upcoming": true
     },
+    {
+      "name": "3.0",
+      "branchName": "3.0.x",
+      "slug": "3.0",
+      "current": true
+    },
+    {
+      "name": "2.2",
+      "branchName": "2.2.x",
+      "slug": "2.2",
+      "maintained": true
+    },
     {
       "name": "2.1",
       "branchName": "2.1.x",
       "slug": "2.1",
-      "current": true
+      "maintained": false
     },
     {
       "name": "2.0",