[TASK] Remove mention of template.php

template.php is removed from core. Rename this to
\TYPO3\CMS\Backend\Template\DocumentTemplate

Change-Id: Ie8378f4951abdfc825c1222861253228ae66e350
Resolves: #48372
Releases: 6.2
Reviewed-on: https://review.typo3.org/20883
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters