Change stale bot to only touch pull requests over 120 days old (#12217)

.github/stale.yml

@@ -1,4 +1,4 @@
-daysUntilStale: 60
+daysUntilStale: 120
 daysUntilClose: 7
 exemptLabels:
   - security
@@ -7,3 +7,4 @@ markComment: >
   This issue has been automatically marked as stale because it has not had
   recent activity. It will be closed if no further activity occurs. Thank you
   for your contributions.
+only: pulls