ci(mergify): upgrade configuration to current format

.mergify.yml

@@ -1,23 +1,20 @@
 queue_rules:
   - name: automerge
-    conditions: []  # No additional rules, because branch-protection does them.
-
-pull_request_rules:
-  - name: automatic merge on CI success and review
-    conditions:
+    queue_conditions:
       - base=main
       - "#approved-reviews-by>=1"
       - label=automerge
       - label!=on hold
-      # GitHub branch-protection rules are automatically applied by mergify, so
-      # the queue can't actually merge things unless _all_ statuses are passed.
-      # This section is just a sanity check before the PR can enter the main
-      # queue.
       - or:
-        - check-success=tests_linux (3.10, ubuntu-latest)
-        - check-neutral=tests_linux (3.10, ubuntu-latest)
-        - check-skipped=tests_linux (3.10, ubuntu-latest)
+          - check-success=tests_linux (3.10, ubuntu-latest)
+          - check-neutral=tests_linux (3.10, ubuntu-latest)
+          - check-skipped=tests_linux (3.10, ubuntu-latest)
+    merge_conditions: [] # No additional rules, because branch-protection does them.
+
+    merge_method: squash
+
+pull_request_rules:
+  - name: automatic merge on CI success and review
+    conditions: []
     actions:
       queue:
-        name: automerge
-        method: squash