configs/basic-pbx: Modified main IVR to play new Allison prompt.

The main IVR was playing demo-congrats. I've switched it over to the
basic-pbx-ivr-main file that we added in core sounds 1.4.27. This prompt
has Allison prompting the user with the actual IVR menu.

ASTERISK-24892 #close

Change-Id: Ifb749616ff8e156a1031ddaddfcc9244767a095d