Do not support deprecated versions of PHP

[dcarbone]

Versions of PHP prior to 8.1 are no longer supported: https://www.php.net/supported-versions.php

Attempting to maintain code compatibility with versions of PHP older than 8.0 is unfeasible at this time. I have also decided to drop support for 8.0 as it is also EOL, and this library makes heavy use of "enums", which may now be implemented directly using the php 8.1+ enum type.

To that end, this library will, as of v3, drop support for all versions of PHP older than 8.1.0.