Exclude on-hold PRs and issues on stalebot. (#1696)

.github/stale.yml

@@ -8,7 +8,8 @@ daysUntilStale: 30
 daysUntilClose: 7
 
 # Issues or Pull Requests with these labels will never be considered stale. Set to `[]` to disable
-exemptLabels: []
+exemptLabels:
+  - on-hold
 
 # Set to true to ignore issues in a project (defaults to false)
 exemptProjects: false