I noticed, that the push mirrors endpoint, is the only endpoint which
returns the times in long format rather than as time.Time().
I think the behavior should be consistent across the project.
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## ⚠️ BREAKING ⚠️
This PR changes the time format used in API responses for all
push_mirror endpoints which return a push mirror.
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Co-authored-by: Giteabot <teabot@gitea.io>