[BUGFIX] Revert breaking change in BackendLayout class

While cleaning up BackendLayout code, a
"declare(strict_types=1);" line was introduced,
which made existing v13-compatible PHP code
break. This is now reverted in order to smoothen
the upgrade path to v13 LTS.

Resolves: #104099
Related: #103365
Releases: main
Change-Id: Ic0de7e903da365178d5e417ff5eeb60c00f1b516
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84701


Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Georg Ringer <georg.ringer@gmail.com>