Add compatability with php8.4

[craig-mcmahon]

This change prevents deprecations warnings such the following occurring when running on php 8.4

PHP Deprecated:  QuickBooksOnline\API\Core\OAuth\OAuth2\OAuth2LoginHelper::__construct(): Implicitly marking parameter $serviceContext as nullable is deprecated, the explicit nullable type must be used instead

This change requires php 7.1, but given that there is a requirement on guzzlehttp/guzzle version ^7.9 this implies a minimum version of php 7.2.5 which I have replicated in composer.json to make it clearer