This is needed to screen readers know that the Select2 is disabled
when focus is put on the selection container. Because we were
mirroring the disabled state to the search input on a multiple
select in the past, this is really only needed for single select
elements which would not otherwise has the disabled property.
This was identified in a previous accessibility audit as being
something which Select2 did not properly report because we were
not setting the attributes properly.
Fixes#4575
89576153a0