ci(mergify): upgrade configuration to current format

Author: mergify[bot]

.mergify.yml

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