[PHP 8.1] Deprecated function: Return type of Drupal\external_link_popup\Entity\ExternalLinkPopup::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed

Problem/Motivation

In PHP 8.1 a tentative return type was added to the method: JsonSerializable::jsonSerialize()

These trigger a deprecation warning on existing code. Code that is compatible with PHP 8.1 only can add the return type now, for backward compatibility we can declare #[ReturnTypeWillChange] which silences the warning and tells PHP we will add the return type later on.

Proposed resolution

Add #[ReturnTypeWillChange] to jsonSerialize method.