[BUGFIX] Fix wrong function calls on AbstractGridObject in hook code

The fluidbased page layout refactoring done in #90882 missed two spots,
where the no longer existing function getBackendLayout() is called on
a GridObject.

Resolves: #91162
Related: #90882
Releases: master
Change-Id: I7d381fe35a02c4f1464f9c4bb9fc31b27ef02b1d
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64297


Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Josef Glatz <josefglatz@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Josef Glatz <josefglatz@gmail.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>