fix(dropdown): trigger button should never be disabled

[nikkimk]

What I did

1. Removed disabled from toggle button.

Testing Instructions

1. In Dropdown docs -> disabled, use a keyboard to navigate to toggle the dropdown.
2. Verify the menuitems can be read, by a screenreader but are disabled.

Notes to Reviewers

1. WAI-ARIA recommends elements of a larger composite widget remain focusable.