Drop support for PHP 5 ?

[ksubileau]

With the recent release of PHP 8, the 5.x branch is getting really old, so it's time to embrace the cool new features of PHP>=7!

• [x] Stop testing against PHP 5
• [x] Remove code specifically dealing with PHP 5 if any
• [x] Add PHP 7 specific syntax features like type-hinting
• [x] Require PHP 7.0 or even 7.2 or higher
• [x] Upgrade PHPUnit