fix(php): suppress notice of JsonSerializable::jsonSerialize

[ChristophWurst]

Fixes

 > phpunit -c tests/phpunit.xml tests/Unit
PHPUnit 9.6.3 by Sebastian Bergmann and contributors.

Runtime:       PHP 8.2.28
Configuration: tests/phpunit.xml
Warning:       No code coverage driver available
Warning:       Your XML configuration validates against a deprecated schema.
Suggestion:    Migrate your XML configuration using "--migrate-configuration"!

PHP Fatal error:  During inheritance of JsonSerializable: Uncaught Return type of OCA\SuspiciousLogin\Service\Statistics\TrainingDataStatistics::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice

/home/runner/work/suspicious_login/suspicious_login/apps/suspicious_login/lib/Service/Statistics/TrainingDataStatistics.php:42
/home/runner/work/suspicious_login/suspicious_login/apps/suspicious_login/lib/Service/Statistics/TrainingDataStatistics.php:14
/home/runner/work/suspicious_login/suspicious_login/apps/suspicious_login/tests/Unit/Service/AdminServiceTest.php:73
 in /home/runner/work/suspicious_login/suspicious_login/apps/suspicious_login/lib/Service/Statistics/TrainingDataStatistics.php on line 14
Script phpunit -c tests/phpunit.xml tests/Unit handling the test:unit event returned with error code 255
...
Error: Process completed with exit code 255.

on CI

[ChristophWurst]

/backport to stable31

[ChristophWurst]

/backport to stable30