update syntax to php 8.4

[MathHRM]

php deprecated the use of implicity marking parameter as null, now it is giving some advising

PHP Deprecated:  Saloon\RateLimitPlugin\Limit::__construct(): Implicitly marking parameter $responseHandler as nullable is deprecated, the explicit nullable type must be used instead in /app/vendor/saloonphp/rate-limit-plugin/src/Limit.php on line 72
PHP Deprecated:  Saloon\RateLimitPlugin\Limit::exceeded(): Implicitly marking parameter $releaseInSeconds as nullable is deprecated, the explicit nullable type must be used instead in /app/vendor/saloonphp/rate-limit-plugin/src/Limit.php on line 126

[Sammyjo20]

This has now been fixed by #18