Add PHP 8.4 support

[mvorisek]

WAITING ON spatie/async#236

needs https://github.com/spatie/async/blob/1.6.1/src/Runtime/ParentRuntime.php#L29 fixed first

1 test triggered 1 PHP deprecation:

1) /__w/lock/lock/vendor/spatie/async/src/Runtime/ParentRuntime.php:29
Spatie\Async\Runtime\ParentRuntime::init(): Implicitly marking parameter $autoloader as nullable is deprecated, the explicit nullable type must be used instead

Triggered by:

* malkusch\lock\Tests\mutex\MutexConcurrencyTest::testHighContention#flock
  /__w/lock/lock/tests/mutex/MutexConcurrencyTest.php:94