Return RFC3339 UTC timestamps for machine-readable output (#470)

### ⚠️ breaking changes ⚠️

- unset timestamps will not be printed as `"0001-01-01 00:00"`, but as empty value `""`
- output formats `csv`, `tsv`, `yaml` output timestamps in UTC instead of local time, and adhere to [RFC3339](https://datatracker.ietf.org/doc/html/rfc3339)

Co-authored-by: Norwin <git@nroo.de>
Co-authored-by: 6543 <6543@obermui.de>
Reviewed-on: https://gitea.com/gitea/tea/pulls/470
Reviewed-by: 6543 <6543@obermui.de>
Reviewed-by: John Olheiser <john.olheiser@gmail.com>
Co-authored-by: Norwin <noerw@noreply.gitea.io>
Co-committed-by: Norwin <noerw@noreply.gitea.io>

modules/print/repo.go

@@ -124,7 +124,7 @@ func (x printableRepo) FormatField(field string, machineReadable bool) string {
 	case "ssh":
 		return x.SSHURL
 	case "updated":
-		return FormatTime(x.Updated)
+		return FormatTime(x.Updated, machineReadable)
 	case "url":
 		return x.HTMLURL
 	case "permission":