[General] Update issue tracker's duplicate resolution message (#47981)

Reopens the change from #46743 (which appears to be broken) on a fresh
branch.

Original author: @daverayment

## Summary

GitHub newcomers can be confused by the current duplicate resolution
message, as it doesn't clearly point to the original referenced issue -
see
https://github.com/microsoft/PowerToys/issues/46347#issuecomment-4103681050.
They may not realise that the #12345 in the duplicate comment is the
relevant link.

This small wording update to the duplicate resolution message tightens
up wording slightly and includes reference to the prior `/dup #nnn`
comment so newcomers don't miss it.

### Before
> Hi! We've identified this issue as a duplicate of another one that
already exists on this Issue Tracker. This specific instance is being
closed in favor of tracking the concern over on the referenced thread.
Thanks for your report!

### After
> We've identified this issue as a duplicate of an existing one and are
closing this thread so discussion stays in one place.<br/><br/>Please
see the comment above for the link to the original tracking issue, and
feel free to subscribe there for updates.

## Validation Steps Performed
N/A - bot reply text change only.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>