PHP 8.4 compatibility [#3573932]

Problem/Motivation

Fix warnings like:

Deprecated function: Drupal\background_image\BackgroundImageInterface::associateEntity(): Implicitly marking parameter $entity as nullable is deprecated, the explicit nullable type must be used instead in include() (line 576 of /var/www/new-diagnostics.com/vendor/composer/ClassLoader.php)

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Issue fork background_image-3573932

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3573932-php-8.4-compatibility changes, plain diff MR !10
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