Add backlog label to be excluded by stale bot

Author: simPod

.github/stale.yml

@@ -4,8 +4,8 @@ daysUntilStale: 30
 daysUntilClose: 7
 # Issues with these labels will never be considered stale
 exemptLabels:
-    - pinned
     - security
+    - backlog
 # Label to use when marking an issue as stale
 staleLabel: wontfix
 # Comment to post when marking an issue as stale. Set to `false` to disable