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>

templates/swagger/v1_json.tmpl

@@ -21646,6 +21646,7 @@
       "properties": {
         "created": {
           "type": "string",
+          "format": "date-time",
           "x-go-name": "CreatedUnix"
         },
         "interval": {
@@ -21658,6 +21659,7 @@
         },
         "last_update": {
           "type": "string",
+          "format": "date-time",
           "x-go-name": "LastUpdateUnix"
         },
         "remote_address": {