ci(mergify): upgrade configuration to current format

.mergify.yml

@@ -1,15 +1,14 @@
 queue_rules:
-    - name: default
-      merge_conditions:
-          - check-success=buildbot/nix-eval
-defaults:
-    actions:
-        queue:
-            merge_method: rebase
+  - name: default
+    queue_conditions:
+      - base=main
+      - label~=merge-queue|dependencies
+    merge_conditions:
+      - check-success=buildbot/nix-eval
+    merge_method: rebase
+
 pull_request_rules:
-    - name: merge using the merge queue
-      conditions:
-          - base=main
-          - label~=merge-queue|dependencies
-      actions:
-          queue:
+  - name: refactored queue action rule
+    conditions: []
+    actions:
+      queue: