Ticket feature: issue/pr links in ticket comment not correct

[petersutter]

What happened:
When a ticket comment includes a reference like #<issue-nr>, the markdown renders a link to https://github.com/gardener/dashboard/issues/<issue-nr>. It should instead link to the ticket repository, e.g. https://enterprise.github.com/ticket-org/ticket-repo/<issue-nr>.

What you expected to happen:

How to reproduce it (as minimally and precisely as possible):

Anything else we need to know:

Environment:

[gardener-ci-robot]

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• After 90d of inactivity, lifecycle/stale is applied
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