Replaced getenv calls with env() in the paystack.php configuration file to fetch environment variables.

Issue with getenv() in Production: The original implementation using getenv() was not loading environment variables correctly in the production environment. This caused issues such as the error "URI must be a string or UriInterface," as the required configuration values were not properly retrieved.env() for Consistency: Using env() aligns with Laravel’s recommended practices for accessing environment variables. It ensures that values are correctly loaded from the .env file and properly utilized in the application.Configuration Caching: The env() function allows Laravel to cache configuration settings, improving performance and ensuring that environment-specific values are consistently available across different environments.