[BUGFIX] Add ext-libxml to composer.json

Some system extensions use functions like libxml_disable_entity_loader
which are provided by ext-libxml. To make sure these methods are
available, this PHP extension needs to be required.

Used composer command:

  composer require ext-libxml:"*"

Also add the extension as required in the install tool.

Releases: master,10.4
Resolves: #91069
Change-Id: Ib98d4aba4a9071f48e2fb370382f61130115daad
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64204


Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>