Support for PHP 8.1

[sal39]

It appears that the composer.json specifies that only PHP 7.2 is supported by version 4.0.0. If Slim-Validation works with other version such as 8.0 or 8.1 this should be updated to allow other php versions. If the software does not support PHP 8, perhaps that can be added to the release notes or readme.

"require": {
"php": "^7.2"

Thanks for making this code available.