Deprecated function: CSSSnippet::jsonSerialize()

Problem/Motivation

Using Background Image Field 8.x-1.7 with Drupal 9.4.3 and PHP 8.1, I get this notice:

Deprecated function: Return type of Drupal\bg_img_field\Component\Render\CSSSnippet::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in include() (line 10 of modules/contrib/bg_img_field/src/Component/Render/CSSSnippet.php).