Improve work item detail accessibility

What does this MR do and why?

This MR fixes some of the work item detail accessibility issues by

1. Adding accessible names to the assignees and labels inputs
2. Improving the contrast of grey text

Changelog: fixed

Screenshots or screen recordings

before	after

How to set up and validate locally

1. In rails console enable the feature flag

   Feature.enable(:work_items)
   Feature.enable(:work_items_mvc_2)

2. In an issue, create a task in the Task widget then click on it
3. Test the MR

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.