Add support for PHP 8, Guzzle 7, Symfony 6, and PHP CS Fixer 2.19

[mcaskill]

Added:

• Support for PHP 8 (resolves #26)
• Support for guzzlehttp/guzzle 7 (resolves #27)
• Support for symfony/console 6
• Support for symfony/phpunit-bridge 6

Removed:

• Dependency on phpunit/phpunit (provided by symfony/phpunit-bridge)

Changed:

• Replaced usage of phpunit binary with simple-phpunit for improved compatibility across phpunit versions.
• Replaced @expectedException annotations with equivalent methods for improved compatibility across phpunit versions.
• Updated friendsofphp/php-cs-fixer to 2.19 (to support PHP 8 while maintaining support for PHP 5).
• Updated initialization of PhpCsFixer\Config object to match requirements for PHP CS Fixer 3.0.
• Renamed .php_cs.dist to .php-cs-fixer.dist.php to match requirements for PHP CS Fixer 3.0.
• Switched build status badge from travis-ci.org to travis-ci.com.

---

In the meantime, people can use my fork:

"require": {
    "mremi/url-shortener": "^2.4"
},
"repositories": [
    {
        "type": "vcs",
        "url": "https://github.com/mcaskill/UrlShortener"
    }
]

[mremi]

Thanks a lot @mcaskill !

[mcaskill]

Thank you.

For Travis CI, you will need to make the switch from travis-ci.org to travis-ci.com in order to restore builds.

The CHANGELOG will need to updated as well to reflect the new version.