Update stale.yml (#4466)

* Disable autoclosing of stale issues/PRs
* Limit stale bot to PRs
See also https://github.com/probot/stale#usage

.github/stale.yml

@@ -1,7 +1,7 @@
 # Number of days of inactivity before an issue becomes stale
 daysUntilStale: 30
 # Number of days of inactivity before a stale issue is closed
-daysUntilClose: 14
+daysUntilClose: false
 # Issues with these labels will never be considered stale
 exemptLabels:
   - pinned
@@ -13,6 +13,8 @@ staleLabel: waiting
 # Comment to post when marking an issue as stale. Set to `false` to disable
 markComment: >
   This issue hasn't seen any activity in the past 30 days. It will be automatically closed if no further activity occurs in the next two weeks. 
+# Limit to only `issues` or `pulls`
+only: pulls
 # Comment to post when closing a stale issue. Set to `false` to disable
 closeComment: false
 # Limit the number of actions per hour, from 1-30. Default is 30