update stale bot per RFC 0124

.github/stale.yml

@@ -1,10 +1,9 @@
 # Configuration for probot-stale - https://github.com/probot/stale
 daysUntilStale: 180
-daysUntilClose: 365
+daysUntilClose: false
 exemptLabels:
   - "critical"
+  - "never-stale"
 staleLabel: "stale"
-markComment: |
-  I marked this as stale due to inactivity. → [More info](https://github.com/NixOS/nix/blob/master/.github/STALE-BOT.md)
-closeComment: |
-  I closed this issue due to inactivity. → [More info](https://github.com/NixOS/nix/blob/master/.github/STALE-BOT.md)
+markComment: false
+closeComment: false