Remove PHP 7.1 for future releases & fix dependency hell

[asgrim]

Future releases will not support PHP 7.1. Existing releases that are installable on PHP 7.1 will continue to work, and receive security fixes where needed.

At the same time I'd like to untangle the dependency hell we're in, which should hopefully become a little easier once we remove PHP 7.1 support.

The Composer platform needs to be added back in (e.g. see previous discussion #243 (comment) ) after it was removed in 1d90c53 to make the project actually installable on 7.1

IMPORTANT NOTE

ScoutAPM will continue to be installable on PHP 7.1. ScoutAPM will continue to work on PHP 7.1. The 8.2.* branch will continue to receive security fixes, but new features will only be available in 8.3.0 and above. We highly recommend upgrading to the latest version of PHP, but understand that it is not always possible, so we will continue to support the 8.2.* branch for now.