Turn off Shirriff debug mode. (#3945)
Turn off Shirriff debug mode. (#3945)

* Add 'Shirriff' action to manage stale issues and PRs.

Creates an Action named 'Shirriff' which runs the GitHub "stale"
action every day at 01:00 UTC. The action will mark issues older
than 14 days as 'status/stale' and add a comment. It will not
check issues with the 'type/epic' or 'status/icebox' labels.

This first iteration is debug-only. If the dry-run looks good we'll
turn off debug mode.

* Turn off Shirriff debug mode. Adjust days-before-stale to 21.
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