Changes default text to "Unassigned"

What does this MR do?

Change the default text of the dropdown in a new issue for for assignees to be the same gray color as the rest.

Are there points in the code the reviewer needs to double check?

That it does in fact change the default text to gray.

Why was this MR needed?

According to https://gitlab.com/gitlab-org/gitlab-ce/issues/32802 the text was not gray

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #32802 (closed)