Activate stale bot (#9272)

substrate/.github/stale.yml

+# 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
+# Issues with these labels will never be considered stale
+exemptLabels:
+  - "D9-needsaudit 👮"
+# Label to use when marking an issue as stale
+staleLabel: "A3-stale"
+# Comment to post when marking an issue as stale. Set to `false` to disable
+markComment: >
+  Hey, is anyone still working on this? Due to the inactivity this issue has
+  been automatically marked as stale. It will be closed if no further activity
+  occurs. Thank you for your contributions.
+# Comment to post when closing a stale issue. Set to `false` to disable
+closeComment: false