[11.x] add Generics to Paginator's ArrayAccess methods

[taka-oyama]

This PR will make Paginator aligned with Collection, which already has these generic types defined for ArrayAccess methods.

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If applicable, please consider releasing your code as a package so that the community can still take advantage of your contributions!