Laravel 8.x compatibility

[antimech]

Our library needs to be keep up to date with industry. Here is some things we need to do:

• [x] Opt for PHP ^7.3|^8.0 composer dependency
• [x] Opt for PHPUnit ^9.3.3 composer dependency
• [x] Opt for Testbench ^6.0.0 composer dependency
• [x] Opt for Guzzle ^7.0.1 composer dependency
• [x] Remove Laravel 5 from tags in composer.json as it's gonna be compatible with newer versions

Each dependency requires to follow it's upgrade guide to avoid breaking changes. All tests must pass.

PRs are welcome!

[antimech]

@AlaaElrifaie thank you for your interest! Merged.

The release will soon be updated on Packagist. Until then you can use this:

"repositories": [
    {
        "type": "git",
        "url": "https://github.com/shakurov/laravel-coinbase.git"
    }
],
"require":{
    "shakurov/laravel-coinbase": "^0.7.7"
}