Refactor the array combination functions in `CSSBlockList` using `array_push`

[oliverklee]

… and the splat operator.

https://www.php.net/manual/en/function.array-push.php

We might need to raise the minimum PHP version first.

[JakeQZ]

… and the splat operator.

We might need to raise the minimum PHP version first.

PHP 7.2 is the minimum version in the main branch; this supports most splattable features, including being able to specify strict types. I don't expect we would need 'first class callables', which require PHP 8.1, and would I think be a bump too far. Some good information in answers here: https://stackoverflow.com/questions/41124015/what-is-the-meaning-of-three-dots-in-php.

[oliverklee]

We'll need at least PHP 7.3 for this - in #1045, the 7.2 unit tests are failing.

[JakeQZ]

We'll need at least PHP 7.3 for this - in #1045, the 7.2 unit tests are failing.

This is presumably for array_push to be called with only one argument.