Support Laravel 9 [new major version required]

[patrickomeara]

• Laravel 9 requires PHP 8 or above so it's time for a major version bump for this package
• I have added info on using anything less than Laravel 8 or PHP 8 in the README

[patrickomeara]

For anyone wanting to use this before it's merged, add this to your composer.json

    "require": {
        "divineomega/laravel-last-activity": "dev-master"
    },    
    "repositories": [
        {
            "type": "vcs",
            "url": "https://github.com/patrickomeara/laravel-last-activity"
        }
    ]