PHPCS/Composer: update PHPCompatibility

[jrfnl]

Composer:

• wimg/php-compatibility has been abandoned for nearly a year. Use phpcompatibility/php-compatibility instead.
• Use the latest version of PHPCompatibility. You were missing out on a lot of new checks, including the checks to make sure your code is compatible with the upcoming PHP 7.4.
• Use the latest version of the DealerDirect Composer PHPCS plugin. Composer treats minors < 1.0 as majors, so you need to explicitely update.

PHPCS ruleset:

• Actually check for cross-version compatibility. You were currently just checking if the code was compatible with PHP 5.6, not with PHP 5.6 and above.

Refs:

• https://github.com/PHPCompatibility/PHPCompatibility#sniffing-your-code-for-compatibility-with-specific-php-versions
• https://github.com/PHPCompatibility/PHPCompatibility/releases/
• https://github.com/Dealerdirect/phpcodesniffer-composer-installer/releases

[zytzagoo]

Thanks a ton for your contribution!