[TASK] Deprecate TypoScript config.xhtmlDoctype

The "new" property config.doctype has been introduced in TYPO3 v6
and should be used now.

It's time to unify all of our settings.

Resolves: #100461
Releases: main
Change-Id: I816ee4e9b2188626808a4bd2411eb64d801e5a03
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78303


Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>