ci: Respect do not merge tag in mergify (#2465)

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

.github/mergify.yml

@@ -5,6 +5,8 @@ queue_rules:
 pull_request_rules:
   - name: Automatic merge on approval
     conditions:
+      and: 
+        - "label!=do-not-merge"
         - and:
           - "#review-threads-unresolved=0"
           - "#approved-reviews-by>=2"