Make selected option for <MultiSelect> and <SearchMultiSelect> consistent

[jscottsmith]

Property for indicating selection is inconsistent.

<SearchMultiselect> uses isSelected
<Multiselect> uses selected

We should pick one over the other and deprecate. The question is which do we prefer? :thonk:

[edrpls]

I think we usually use the is- prefix for almost everything, so that would be consistent with our codebases in general.

Should the PR should support both methods and warn about deprecation, or remove the old one altogether and be released as a minor update? If we support both I guess it could be just a patch?

[jscottsmith]

I prefer isSelected too.

Should the PR should support both methods and warn about deprecation.

Yes. I'm thinking a patch makes this an easy update.

[edrpls]

Yep, that sounds good to me.