[!!!][TASK] Use correct ISO8601 format

DateTime::ISO8601 is not compatible with ISO-8601, but is left this
way for backward compatibility reasons. Use DateTime::ATOM or
DATE_ATOM for compatibility with ISO-8601 instead.

Resolves: #77919
Releases: master
Change-Id: Iab7217c492b4137b8cb0b6e3608e2d8071d861f6
Reviewed-on: https://review.typo3.org/49906


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Jan Helke <typo3@helke.de>
Tested-by: Jan Helke <typo3@helke.de>
Tested-by: Tomasz Stępniak <stempl88@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>