Update composer.json in order to support PHP 8.0

[Alex]

Hi,

I was updating my app to support PHP 8.0 runtime and i've got this issue on composer install :

Problem 1
    - spatie/laravel-referer is locked to version 1.7.0 and an update of this package was not requested.
    - spatie/laravel-referer 1.7.0 requires php ^7.2 -> your php version (8.0.0) does not satisfy that requirement.

I'll make a pull request in order to support PHP 8.0

Thanks and have a nice day !