Drop support for older PHP versions

[oliverklee]

I propose this library drops support for older PHP versions so it can use newer PHP features.

According to https://www.php.net/supported-versions.php, PHP 7.2 recently reached its end of life. So I propose to drop support for all PHP versions < 7.2.

(We then can use Rector to upgrade the code accordingly.)

What do you think?

[oliverklee]

And in https://github.com/MyIntervals/emogrifier, we'd like to use this library. Our next major release will still support PHP 7.2, so PHP-CSS-Parser still supporting PHP 7.2 will be helpful.

[westonruter]

As I commented at #206 (comment), updating the PHP requirement detrimental for WordPress which still supports PHP 5.6. If PHPDoc can be used until WordPress increases the minimum, this would be ideal. Otherwise, the dozens of WordPress plugins that use PHP-CSS-Parser would be forced to fork the package.

[oliverklee]

Then I propose we start with raising the minimum required PHP version to 5.6. What do you think?

[westonruter]

Yes, that works for me!

[oliverklee]

Okay then, 5.6.20 it is (which are the current WordPress PHP requirements). I will create a PR after some of my other PRs are merged.