[10.x] Update collection docs to use template-covariant for the values

[Rocksheep]

This PR will allow developers to use covariants in their collections. Before this was not possible and you would get errors from PHPStan. This is also an issue in older versions of Laravel. The following example would throw PHPStan errors

class Animal {}
class Tiger extends Animal {}
class Lion extends Animal {}
class Zebra extends Animal {}

class Zoo {
    /**
     * @var \Illuminate\Support\Collection<int, Animal>
     */
    private Collection $animals;

    public function __construct() {
        $this->animals = collect([
            new Tiger,
            new Lion,
            new Zebra,
        ]);
    }

    /**
     * @return \Illuminate\Support\Collection<int, Animal>
     */
    public function getWithoutZebras(): Collection {
        return $this->animals->filter(fn (Animal $animal) => ! $animal instanceof Zebra);
    }
}

The thrown error would be

Property Zoo::$animals (Illuminate\Support\Collection<int, Animal>) does not accept Illuminate\Support\Collection<int, Lion|Tiger|Zebra>.

Changing the @template to @template-covariant fixes this.

[nunomaduro]

@Rocksheep Does PHPStorm and Psalm understand that @template-covariant?

[Rocksheep]

@nunomaduro Yeah! Psalm definitely has support for it as can be read on their documentation and here are screenshots to display that PHPStorm also understands it.

[nunomaduro]

Please don't set the pull request "ready for review" while we are discussing it.

1. Why this change is not being applied to Illuminate/Collections/helpers.php, Illuminate/Database/Eloquent/Collection.php, Illuminate/Database/Eloquent/Casts/ArrayObject.php? Is it intentional?
2. Also, why we need this change on the Illuminate/Contracts/Support/Arrayable.php?

[Rocksheep]

Ah sorry I thought I had pushed it out of the review state because of a push I did earlier. I didn't add it to the database collections because I didn't run into issues with that yet, because I don't usually pull multiple models from the database in a single call.

So I have only changed the files where I ran into the problem and I missed the helper.

And about that arrayable. I thought it would have the same problem, but checking now it doesn't seem so. I will remove it and add it to the collection helpers.

Do you think the other classes you mentioned need the template change?