Removes SetMixin

Builds on the removal of the 2.6 compatibility stuff. This PR removes SetMixin usage in MultipleSelectOption and CheckboxValues and replaces them with MutableSet, fixing up the differences between SetMixin and MutableSet (different abstract methods, missing `update` in MutableSet)

An alternative take (maybe a better one?) would be to not remove SetMixin but have it inherit from MutableSet, and remove the methods provided by MutableSet. That way MultipleSelectOption and MutableSet wouldn't be changed, SetMixin would just be drastically simplified instead.