[4.2] Auth: short arrays syntax

As laravel/framework is for > 5.4, it would be appropriate now to switch to short arrays notation, as this is already the case for laravel/laravel.
In order to find the easiest method to ease review, PR should be properly organized.
This can be achieved in many ways
1PR / package, src and tests in different commits, 1 global PR with split commits, 1 PR for 9/10 packages ? @taylorotwell , let me know