Change the PHP version requirement from 7.1 to 7.4.

[ChadSikorra]

All versions of PHP up to 7.4 are now EOL, and have been for some time:

https://www.php.net/supported-versions.php

Even sometime this year PHP 7.4 will stop getting security updates. This library has supported PHP 7.1+ since I originally released it. I plan to now bump the required version to 7.4+.

I'm planning this to occur in a "1.0.0" version of this library. Older versions of this library will still be available for those using old PHP versions. But staying up-to-date with newer PHP versions will be a good thing. There are lots of PHP 7.4 features that would be good to use (such as typed properties).