.triage-policies.yml: close stale and unassigned issues

Automatically close issues that have been open for more than 6 months
without an assignee and without a linked merge request. Security-labeled
issues are excluded.

The motivation:

- Issues tend to stay open indefinitely under the implicit assumption that
  someone will eventually pick them up. In practice this rarely happens.
- The backlog keeps growing, making it harder to see what is actually
  being worked on.
- The number of contributors who turn an issue into a merge request is
  very small relative to the number of issues filed.
- The goal is to keep the issue tracker focused on work that is actively
  in progress, not as a wishlist.
- Hopefully this encourages a more active contribution culture: instead
  of "I have reported it, someone will pick it up", reporters are nudged
  to either own the fix or accept that it may not happen.

Resolves: #740
Signed-off-by: Nikos Mavrogiannopoulos <n.mavrogiannopoulos@gmail.com>

.triage-policies.yml

@@ -1,6 +1,22 @@
 resource_rules:
   issues:
     rules:
+      - name: Close stale unassigned issues
+        conditions:
+          state: opened
+          date:
+            attribute: updated_at
+            condition: older_than
+            interval_type: months
+            interval: 6
+          forbidden_labels:
+            - security
+          ruby: resource[:assignees].empty? && resource[:merge_requests_count].to_i == 0
+        actions:
+          status: close
+          comment: |
+            {{author}} This issue has been open for over 6 months with no assignee and no linked merge request. We are closing it automatically. If you are still interested in addressing it, please re-open and either assign it to yourself or link a merge request. If this is a security issue, please re-open it and notify a maintainer so it can be marked as such. This is an automated message.
+
       - name: Close old needinfo policy
         conditions:
           date: