DSD-1823: using buttonText in MultiSelect for CheckboxGroup legend value

Fixes JIRA ticket DSD-1823

This PR does the following:

Uses the MultiSelect's buttonText prop value as the internal CheckboxGroup's label text value. The previous hardcoded value had no meaning, was repetitive, and was confusing. This updates gives it more context for screen readers.
Also replace the internal div inside the accordion button to a span for better HTML DOM.

[x] Checked Storybook's "Accessibility" tab for color contrast and other issues.
[x] The feature works with keyboard inputs including tabbing back and forward and pressing space, enter, arrow, and esc keys.
[x] For hidden text or when aria-live is used, a screenreader was used to verify the text is read.
[x] For features that involve UI updates and focusing on DOM refs, focus management was reviewed.
[x] The feature works when the page is zoomed in to 200% and 400%.