PHP 8.1 JsonSerializable::jsonSerialize() return type deprecation warning

[koike-moyashi]

Hi 👋

When running this library on PHP 8.1, several deprecation warnings appear related to JsonSerializable::jsonSerialize() return types:

Environment:

• Moodle 4.5.7
• PHP 8.1.30
• logstore_caliper 4.5.4 (2025060500)

PHP Deprecated:  Return type of IMSGlobal\Caliper\events\Event::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /export/html/admin/tool/log/store/caliper/vendor/imsglobal/caliper/src/events/Event.php on line 40
PHP Deprecated:  Return type of IMSGlobal\Caliper\entities\Entity::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /export/html/admin/tool/log/store/caliper/vendor/imsglobal/caliper/src/entities/Entity.php on line 33
PHP Deprecated:  Return type of IMSGlobal\Caliper\request\Envelope::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /export/html/admin/tool/log/store/caliper/vendor/imsglobal/caliper/src/request/Envelope.php on line 21
PHP Deprecated:  Return type of IMSGlobal\Caliper\util\BasicEnum::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /export/html/admin/tool/log/store/caliper/vendor/imsglobal/caliper/src/util/BasicEnum.php on line 

Updating the methods to specify a return type of mixed resolves the issue.
For example:

// Before
function jsonSerialize() {
    return $this->getValue();
}

// After
function jsonSerialize(): mixed {
    return $this->getValue();
}

Do you have any plans to update this library for PHP 8.1 compatibility?

Thanks for maintaining this project!