Update branch metadata (#11663)

2.20.0 just got released

Author: greg0ire

.doctrine-project.json

@@ -35,17 +35,23 @@
             "slug": "3.0",
             "maintained": false
         },
+        {
+            "name": "2.21",
+            "branchName": "2.21.x",
+            "slug": "2.21",
+            "upcoming": true
+        },
         {
             "name": "2.20",
             "branchName": "2.20.x",
             "slug": "2.20",
-            "upcoming": true
+            "maintained": true
         },
         {
             "name": "2.19",
             "branchName": "2.19.x",
             "slug": "2.19",
-            "maintained": true
+            "maintained": false
         },
         {
             "name": "2.18",