Add probot/stale config (#11740)

.github/stale.yml

+# For now we want to only check for stale pull requests
+only: pulls
+
+# Number of days of inactivity before an issue becomes stale
+daysUntilStale: 60
+
+# Number of days of inactivity before a stale issue is closed
+daysUntilClose: 7
+
+# Issues with these labels will never be considered stale
+exemptLabels:
+  - pinned
+  - security
+
+# Comment to post when marking an issue as stale. Set to `false` to disable
+markComment: >
+   This issue was automatically marked as stale due to a lack of recent activity.
+
+   This isn't a criticism of the PR, just an attempt to keep the PR queue in a happier state.
+   If you'd like to bring this PR back to the attention of the folks on the Metabase team, just comment
+   to prevent it from being closed.
+
+# Comment to post when closing a stale issue. Set to `false` to disable
+closeComment: false