Let GH auto-notify and auto-close stale PRs

Signed-off-by: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Rafael Guterres Jeffman <rjeffman@redhat.com>

.github/stale.yml

@@ -0,0 +1,12 @@
+daysUntilStale: 60
+daysUntilClose: 14
+exemptLabels:
+  - postponed
+staleLabel: stale
+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.
+closeComment:
+  This issue has been automatically closed as stale it has not had recent
+  activity.