mergify: Restrict the addition of "priority-review" label

Mergify rules doesn't check the status of pull requests before adding
the "priority-review" label. In addition to the existing rules it only
makes sense to add the label if the pull request is in open state.

Signed-off-by: Anoop C S <[email protected]>

Author: anoopcs9

.github/mergify.yml

@@ -77,6 +77,8 @@ pull_request_rules:
     conditions:
       - "updated-at<15 days ago"
       - "-draft"
+      - "-closed"
+      - "-merged"
     actions:
       label:
         add: