Drop PHP <8.1 Support

[JamesAlias]

As PHP 8.1 is already approaching it's EOL I'm considering dropping the support for PHP <8.1.

I know there have been people using this package in production where PHP was still 7.4 when we developed 1.1.0 but I think it's time.

[JamesAlias]

We already have code that is incompatible with PHP <8.0.

We should definitely clean this up, remove support for <8.1 and add supported version information in composer.json and docs.