Fix deprecated messages in PHP 8.1

[Rahmon]

Since the Collection class implements ArrayAccess interface, deprecated messages are thrown in PHP 8.1 when the returned type is not defined.

Example:

Return type of Iconic_WSB_NS\StellarWP\Uplink\Resources\Collection::offsetGet($offset) should either be compatible with ArrayAccess::offsetGet(mixed $offset): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice

This PR adds the returned types. It's important to notice that the void type was added in PHP 7.1.0. If we want to keep compatibility with older versions, we have to replace void with #[\ReturnTypeWillChange] to avoid the deprecated messages.

How to test this change:

1. Update the composer.json

"repositories": [
...
{
      "type": "vcs",
      "url": "https://github.com/Rahmon/uplink.git"
    }
],
"require": {
...
"stellarwp/uplink": "dev-fix/collection-deprecated-warnings",
}

2. Run composer update stellarwp/uplink

[jamesckemp]

Looks good to me. My only concern is using Plugin @borkweb could it ever be anything different?

[borkweb]

Hrm, yeah, it could be Plugin or Service as the return type. Sounds like we'll have to do #[\ReturnTypeWillChange]