[BUGFIX] Use BackendUtility use statement

Due to a faulty backport BackendUtility was used without proper
namespace, leading to a fatal error while editing page properties.
This introduces the use statement for all occurences in the class.

Resolves: #52728
Releases: 6.1, 6.0
Change-Id: I37b9e98513b1dd173e7099bce68a5950b9dd12e1
Reviewed-on: https://review.typo3.org/24658
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn