[TASK] Deprecate DocumentTemplate

Since TYPO3 v7, the new ModuleTemplate API is in place,
which contains a fluid-based and fluent interface possibility
to add DocHeader components and to render content.

Resolves: #90421
Releases: master
Change-Id: I5634ea0427486f37fbbfe3ee00695fc98e52d980
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63290


Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>