[BUGFIX] Properly handle readOnly in InputSlugElement

While the "readOnly" option is not supported
as columns config for type "slug", it might be
automatically set by FormEngine in case the
field is defined with "l10n_display", set to
"defaultAsReadonly".

This is now properly handled.

Resolves: #96096
Related: #89152
Releases: main, 11.5
Change-Id: I3ca0a6a17cc79704a727a6600b211833601dc290
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72412


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>