[BUGFIX] Fix wrong date in RSS feed

RSS feed must use the english date and not a localized one

Change-Id: I32a98b680aa62a2b1c6e71f1a74386749fe16611
Resolves: #55855
Reviewed-on: https://review.typo3.org/28084
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer