[9.x] Widen the type of Collection::unique $key parameter

[NiclasvanEyk]

This was narrowed down to bool, but the docs and implementation
actually support arbitrary values.

As suggested in #40897 I PRed the changes to Collection and other related classes like LazyCollection.

Description:

Currently the $key parameter of the Collection::unique method has the following type annotation:

    /**
     * Return only unique items from the collection array.
     *
     * @param  (callable(TValue, TKey): bool)|string|null  $key
     * @param  bool  $strict
     * @return static
     */
    public function unique($key = null, $strict = false)

So basically it can be

• a function that returns bool
• a string
• nothing at all

I think the first one is a little bit too strict. Looking at https://laravel.com/docs/9.x/collections#method-unique I should be able to return any value to determine uniqueness:

Finally, you may also pass your own closure to the unique method to specify which value should determine an item's uniqueness:

$unique = $collection->unique(function ($item) {
    return $item['brand'].$item['type'];
});
 
$unique->values()->all();
 
/*
    [
        ['name' => 'iPhone 6', 'brand' => 'Apple', 'type' => 'phone'],
        ['name' => 'Apple Watch', 'brand' => 'Apple', 'type' => 'watch'],
        ['name' => 'Galaxy S6', 'brand' => 'Samsung', 'type' => 'phone'],
        ['name' => 'Galaxy Gear', 'brand' => 'Samsung', 'type' => 'watch'],
    ]
*/