Fix: #11 - Add compatibility for PHP 8.4. Drop compatibility with PHP 7.0

[fisharebest]

This change adds compatibility with PHP 8.4.

Specifically, the deprecation notices about "implicit null parameters". The code has been updated to use "explicit null".

This is a one-line, one-character change.

This fix requires PHP syntax which is not compatible with PHP 7.0, so I have raised the minimum supported version from PHP 7.0 to PHP 7.1.

I've updated the test scripts and CI to work with a wide range of PHP and PHPUNIT versions. Hence the dupliation in the docblock / attributes.

[oscarotero]

Please, change it to ^7.1|^8.0 so it's compatible with future versions (8.5 and upper)

[fisharebest]

Please don't remove 7.0 compatibility.

To add compatibility with PHP 8.4, we must change (Foo $foo = null) to (?Foo $foo = null).

But this syntax is not compatible with PHP 7.0.

The PHP RFC https://wiki.php.net/rfc/deprecate-implicitly-nullable-types says that the required code-change is only available in PHP 7.1

I cannot think of any other solution.

Please, change it to ^7.1|^8.0 so it's compatible with future versions (8.5 and upper)

How can you know that it will be compatible with PHP 8.5?

When PHP 8.5 is released, create a new release with an updated dependency?

[oscarotero]

How can you know that it will be compatible with PHP 8.5?

PHP is semver, it doesn't introduce breaking changes between major versions. So anything that works on 8.4 should work on 8.5.
In fact, the latest version of this library was released 4 years ago and only now needs some minor tweaks to make it work with 8.4.