[Bug] Don't mark due date in merged/ closed PRs as overdue

[delvh]

• Gitea version (or commit ref): 1.14.0+dev-646-g6064bfa3c
• Can you reproduce the bug at https://try.gitea.io:
  • Yes

Description

Currently when a PR has a due date and gets merged or closed, it will, after the due date has expired, get displayed as overdue, even if it was merged on time. This always gets confusing when looking at closed PRs.
The question is: Is it really needed to display the due date when a PR has been closed (independent from if it was closed or merged)?
I'd argue that leaving it out for any closed/ merged PR would suffice to solve the problem, as there is no real value in leaving it in.
Another possible approach would be to display the due date in a PR merged on time normally, and otherwise overdue. This approach however is most likely more complex to implement.
I'm fine with both.
...

Screenshots

[delvh]

Furthermore, the same behavior could also be found when looking explicitly at a closed PR: The due date will also after expiring, no matter whether the PR has been merged/ closed or not, get displayed as overdue.
Here however, the solution will be more complicated as it is counterproductive to completely remove the due date here. Here, we really have to change appearance depending on whether the PR has been merged on time or not.

(the screenshot has been taken on a merged PR (and with German locale…))