Pivot mergify config to use a merge queue (#6211)

In #6188 and #6117 we changed the mergify configuration to use the smart
strict mode which silently maintains a merge queue for us. This option
has recently been superseded by building explict merge queues that
enable more rich control over how PRs to merge get queue together. [1]
This commit switches the mergify config to use an explicit queue for
merging to give us more explicit control over how mergify is going to
merge PRs for us. This also opens up new options if we were to have a
mergify subscription at a future date, primarily speculative testing on
the merge queue [2] which will enable better throughput because it
validates the potential future state all at once and lets us run testing
in parallel.

[1] https://docs.mergify.io/actions/queue/
[2] https://docs.mergify.io/actions/queue/#speculative-checks

.mergify.yml

@@ -1,14 +1,19 @@
+queue_rules:
+  - name: automerge
+    conditions:
+      - check-success=Qiskit.qiskit-terra
+
 pull_request_rules:
   - name: automatic merge on CI success and review
     conditions:
-      - status-success=Qiskit.qiskit-terra
+      - check-success=Qiskit.qiskit-terra
       - "#approved-reviews-by>=1"
       - label=automerge
       - label!=on hold
     actions:
-      merge:
+      queue:
+        name: automerge
         method: squash
-        strict: smart+fasttrack
   - name: backport
     conditions:
       - label=stable backport potential