HPCC-12784 Output WU Stats TimeStamp in UTC format

The existng code does return a GMT time. But, it
should be YYYY-MM-DDThh:mm:ss.xxxZ.

Signed-off-by: wangkx <kevin.wang@lexisnexis.com>
Merge pull request #6792 from wangkx/h12784

HPCC-12784 Output WU Stats TimeStamp in UTC format

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>