Fix deprecated get_class() usage in Router for PHP 8+ compatibility

[chikeh]

Changes made by this pull request

This pull request updates the Router class to fix a deprecation warning in PHP 8.3:

Deprecated function: Calling get_class() without arguments is deprecated in Yabacon\Paystack\Helpers\Router->__construct()

Change summary:

Replaces get_class() with get_class($this) in the Closure::bind() call in Router.php.

Ensures compatibility with PHP 8 and later, where calling get_class() without an argument is deprecated.

This change is backward compatible and resolves the warning encountered during Paystack checkout integration on modern PHP versions.