[TASK] Remove function usage ctype_alnum in PageRenderer

This fixes the problem that on some servers the PHP ctype extension
is not installed by default and results in a blank backend screen
when trying to edit a page.

Fixes: #43726
Releases: 6.1, 6.0

Change-Id: Icb029a39107c092c06c9e70bbde0b2217e5860f7
Reviewed-on: https://review.typo3.org/17017
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn