Issue #2942975: Narrow LayoutEntityDisplayNormalizer::normalize() return type to array

Ivan Shatalovrequested to merge
issue/drupal-2942975:2942975-normalize-return-array into main

Based on !14351.

On Drupal 11.4 ConfigEntityNormalizer::normalize() returns array (https://www.drupal.org/node/3588047). The wider union return type is incompatible.

Closes #2942975

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