Add `.modal-dialog-centered` for optional vertically centered modal (#24510)

Add `.modal-dialog-centered` for optional vertically centered modal (#24510)

* Add .modal-dialog-centered for optional vertically cenetered modal

Fixes #23638

* adds modal-dialog-centered class to docs and removes margin to avoid generating a vertical scrolling

* mention limitations

* fix aria attr

* Add `width: 100%` to the .modal-content for the centered version. Adding it here to avoid adding another selector by limiting it to the centered modal modifier.