ci(mergify): upgrade configuration to current format

.mergify.yml

@@ -1,6 +1,20 @@
 queue_rules:
   - name: default
-    conditions:
+    queue_conditions:
+      - "#approved-reviews-by>=1"
+      - "#changes-requested-reviews-by=0"
+      - base=main
+      - label=checkin-needed
+      - check-success=Decision Task
+      - check-success=run-tests
+      - check-success=ios-test-and-artifacts
+      - check-success=clippy
+      - check-success=check-formatting
+      - "#check-success>=5"
+      - "#check-pending=0"
+      - "#check-stale=0"
+      - "#check-failure=0"
+    merge_conditions:
       # Conditions to get out of the queue (= merged)
       - check-success=Decision Task
       - check-success=run-tests
@@ -9,6 +23,8 @@ queue_rules:
       - "#check-pending=0"
       - "#check-stale=0"
       - "#check-failure=0"
+    merge_method: rebase
+
 pull_request_rules:
   - name: remove outdated reviews for non-core authors
     conditions:
@@ -23,38 +39,6 @@ pull_request_rules:
         remove:
           - checkin-needed
   - name: automatic merge for main
-    conditions:
-      - "#approved-reviews-by>=1"
-      - "#changes-requested-reviews-by=0"
-      - base=main
-      - label=checkin-needed
-      # What we want to say here is "all checks passed", but that's not a concept
-      # that makes sense in GitHub:
-      #
-      #    https://docs.mergify.io/conditions/#validating-all-status-checks
-      #
-      # But we don't want to just list out all the checks, because we have many of
-      # them and some of them are added dynamically and we might add or remove checks
-      # without updating the list here.
-      #
-      # So instead we insist that a small set of known checks has completed,
-      # and also that there are no other checks in unexpected states.
-      - check-success=Decision Task
-      - check-success=run-tests
-      - check-success=ios-test-and-artifacts
-      - check-success=clippy
-      - check-success=check-formatting
-      - "#check-success>=5" # we should always have at least this many checks
-      - "#check-pending=0"
-      - "#check-stale=0"
-      - "#check-failure=0"
+    conditions: []
     actions:
-      # This merges by rebasing without strict merge, which means that mergify will *attempt*
-      # to rebase a matching PR directly on to the `main` branch, even if the PR is out of date.
-      # However, our GitHub branch protection rules require that PRs must be up-to-date before merging,
-      # and mergify respects these rules, so the expected behaviour here is that mergify will only
-      # merge when it can cleanly put the exact commits of the PR directly on top of `main`
-      # (and thus avoid having to re-write any commits or create merge commits of its own).
       queue:
-        method: rebase
-        name: default