Drop unsupported dependencies

[tacman]

This library could take advantage of some of the goodies in PHP 8 by dropping support for PHP 7.

You could tag this as version 1 (for BC), then use rector to refactor and take advantage of better type-hinting, return types, etc.

[samwilson]

Which dependencies are unsupported? composer outdated doesn't report any.

You could tag this as version 1 (for BC)

I'm not sure I understand. This is already at version 5.

But yes, good idea in general: let's permit the use of newer versions. I do want to maintain backwards compatibility if possible (i.e. unless we need to break it in order to support new versions).