Consider making toggleText required for the base dropdown

The following discussion from gitlab-org/gitlab-ui!3218 (merged) should be addressed:

• @sdejonge started a discussion:

  suggestion: add toggleText and textSrOnly props to relevant stories to illustrate labelling. Maybe future work can be to have toggleText required on base dropdown to enforce labels.

This improvement is important for accessibility especially in the case when the custom toggle content has no text so the screen reader does not provide any useful overview for it.