Fix PHP 8.4 implicitly nullable arguments deprecation. Remove support for PHP versions before 7.1.

[artursreiljans]

PHP 8.4 has deprecated implicitly nullable types (function foo(string $bar = null) {} instead of function foo(?string $bar = null) {}). This pull request addresses that.

However, explicitly nullable types were only introduced in PHP 7.1 (https://www.php.net/manual/en/migration71.new-features.php). Thus, this same pull request drops support for versions 5.6 and 7.0.

According to the latest Packagist statistics, their usage is below 1% in 2025 (https://stitcher.io/blog/php-version-stats-january-2025) – and in fact, has already been since at least late 2021 (https://stitcher.io/blog/php-version-stats-january-2022).

I suggest dropping the support for ancient versions and paving the way to 9.0 which is likely to convert current deprecation notices into plain hard fatal errors.

[zanbaldwin]

Thanks for this, I think I going to add create 6.0 for modern PHP, and keep 5.x for legacy support (like has been done for ip-doctrine). As there will be two supported versions active at the same time I'll have to figure out what the branches should look like, but once I do I'll merge this in :D

[zanbaldwin]

Pull request closed automatically when I replaced develop/master branches with 5.x/6.x branches.
I'll try and cherry pick your commits into the new branches so you still get credit 👍