When single select `multiple` attribute is added and removed, it's possible to get a single select with multiple selections.

[anselmbradford]

Describe the bug

The multiple attribute can be added to the <cfpb-select> web component, which turns it into a multiselect. Multiple items can then be selected. If the multiple attribute is then removed, multiple items remain checked. This state change should probably only leave the last selection selected, and unselect all others.