Bump to PHP 8, tighten/collect:^9

[tacman]

Now that php 7.4 is officially EOL, what do you think of dropping support for the older versions?

That would allow for upgrading the library to return typehints, etc.

Or, at a minimum what do you think of requiring ^7.4|^8.0 in composer.json, and tighten/collect:^8.0|^9.0

I can make a PR if you're okay with that. My preference would be for PHP 8.0+, but maybe you'd want that to be a new release.

[tacman]

OK, now collect has been deprecated, it should be replaced with composer require illuminate/collections