Test on PHP 8.4

[sanmai]

What and why? 🤔

Let's start testing the library on PHP 8.4 and fix issues. Specifically, implicit nullable are now deprecated as of PHP 8.4:

- public function __construct(CacheProvider $cache_provider = null)
+ public function __construct(?CacheProvider $cache_provider = null)

Unless we make the change above, we'll see:

Deprecated:   Implicitly marking parameter $cache_provider as nullable is deprecated, the explicit nullable type must be used instead

This is a non-breaking change, as you can see from the lack of LSP errors: https://3v4l.org/VmME0

Testing Steps ✍️

CI should pass, review the changes.

You're it! 👑

Tag a random reviewer by adding @Automattic/stream-builders to the reviewers section, but mention them here for visibility as well.

[coveralls]

coverage: 79.704%. remained the same
when pulling 548cad0 on sanmai/php84
into a1d595e on main.

[sanmai]

We would want to switch to PHP 8.1 for static analysis and other common stages, but there are branch protection rules that demand PHP 7.4. So let's add 8.1 first and tweak the protections later.