[TASK] Use new docheader for backend template

Thanks to Robert Vock for the patch

Resolves: #56091
Releases: 6.2

Change-Id: I36f1b017a80440a2d603877549f7594a8ea4a817
Reviewed-on: https://review.typo3.org/28652
Reviewed-by: Nico de Haen
Tested-by: Nico de Haen