add role="listbox" to .carousel-inner to improve accessibility of Carousels

Refs #13556.
Per https://github.com/paypal/bootstrap-accessibility-plugin/#carousel
Credit: https://github.com/paypal/bootstrap-accessibility-plugin

[skip sauce]
Merge pull request #14071 from twbs/carousel-inner-role-listbox

add role="listbox" to .carousel-inner to improve accessibility of Carousels