feat: Multiple Mergify Queues (#7437)

* Multiple mergify queues.

* Update urgent label

* Use queue conditions as opposed to conditions field

.github/mergify.yml

 queue_rules:
+  - name: urgent
+    queue_conditions:
+      - label = C-urgent
   - name: default
-    conditions: []
+    queue_conditions: []
+  - name: lowpriority
+    queue_conditions:
+      - author=dependabot[bot]
 
 pull_request_rules:
   - name: Automatic merge on approval