ci(mergify): upgrade configuration to current format

.mergify.yml

@@ -1,24 +1,23 @@
 queue_rules:
-  - name: default
-    conditions:
-      - check-success=build
-
-pull_request_rules:
-  - name: Automatic merge
-    conditions:
+  - name: duplicated default from Automatic merge
+    queue_conditions:
       - base=master
       - "#approved-reviews-by>=1"
       - "status-success=build"
-    actions:
-      queue:
-        method: merge
-        name: default
-  - name: Merge scala-steward
-    conditions:
+    merge_conditions:
+      - check-success=build
+    merge_method: merge
+  - name: duplicated default from Merge scala-steward
+    queue_conditions:
       - base=master
       - author=scala-steward
       - "status-success=build"
+    merge_conditions:
+      - check-success=build
+    merge_method: merge
+
+pull_request_rules:
+  - name: Automatic merge + Merge scala-steward
+    conditions: []
     actions:
       queue:
-        method: merge
-        name: default