Use pivot model fromDateTime instead of assuming Carbon

[rudiedirkx]

InteractsWithPivotTable assumes freshTimestamp() is a Carbon instance, but it doesn't have to be. For normal models, that already works, because you can override Model's freshTimestamp and fromDateTime, but the pivot model doesn't use fromDateTime.

In my app I want Laravel to update timestamps with Laravel logic, but I don't want 1000s of Carbon instances everywhere for easy time comparisons, so I store and use ints for timestamps, and I never automagically use Carbon. These 3 method overrides in my app's Model do all of that very simply:

trait MyCustomTimestamps {
	public function freshTimestamp() {
		return time();
	}

	public function asDateTime($value) {
		return $value;
	}

	public function fromDateTime($value) {
		return $value;
	}
}

except for the pivot model's automatic timestamp.

I've been using the same local patch in Laravel 8 - 10, and it still works. This is that PR.