Update work item label widget UX

What does this MR do and why?

1. Resets search when a label is selected to align with existing label behavior
   1. Also updates Assignees to use the same behavior via the shared component
2. Changes dropdown text from "Select label" to "Select labels"
3. Removes floating boolean text

What does this MR not do?

• This does not update the selected labels to use the label name when 1 label
• Does not address dropdown header resizing ( gitlab-ui#2171)

MR acceptance checklist

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Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Run Feature.enable(:work_items_mvc_2) in rails c
2. Create and open a task from an issue

Related to #442678 (closed)