Update PHP dependency to allow installation with PHP 8

[ParhamG]

PHP version is currently locked to ^7.1 in composer.php and is not allowing update/installation of the package in an environment with PHP 8 as a platform requirement. If the Atomic PHP client is already compatible with PHP 8, can we please have the dependency updated to allow PHP 8 as well?

[ParhamG]

This is still blocking installations on PHP 8.X