Clean up Duplicate Surfacing comment noise

[denelon]

The Duplicate Surfacing workflow's comment carries unnecessary noise. Mirror the winget-cli cleanup (microsoft/winget-cli#6412) here so the comment is friendlier and less noisy:

1. Drop the (surfaced automatically for maintainer review — this issue has not been closed) parenthetical from the header.
2. Reword the guidance to address the issue submitter (thank them, ask them to self-review the candidates, and — if one matches — 👍 that issue and close this one as a duplicate). Retain the note that older/curated issues are preferred as canonical.
3. Replace the auto-generated Generated by … · AIC … footer with a consolidated H6 Template: msftbot/duplicate/surfaced by [Duplicate Surfacing](<run-url>) line via safe-outputs.messages.footer.

Keeps the high-confidence threshold and the invisible dedup markers unchanged.