Fix push mirror, wrong timestamp format (#27153)

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>

services/convert/mirror.go

@@ -15,8 +15,8 @@ func ToPushMirror(pm *repo_model.PushMirror) (*api.PushMirror, error) {
 		RepoName:       repo.Name,
 		RemoteName:     pm.RemoteName,
 		RemoteAddress:  pm.RemoteAddress,
-		CreatedUnix:    pm.CreatedUnix.FormatLong(),
-		LastUpdateUnix: pm.LastUpdateUnix.FormatLong(),
+		CreatedUnix:    pm.CreatedUnix.AsTime(),
+		LastUpdateUnix: pm.LastUpdateUnix.AsTimePtr(),
 		LastError:      pm.LastError,
 		Interval:       pm.Interval.String(),
 		SyncOnCommit:   pm.SyncOnCommit,