Improve search bar: active state

What does this MR do?

When the search bar becomes active:

• The width of the search box should increase to 320px for more comfortable typing.
• The width of the suggestions dropdown should be 320px
• The separator between the options for issues and MRs should be removed.
• The headers that indicate the scope should be as follows:
  • Project: Name of the project (no change)
  • Group: Name of the group (no change)
  • Global: All GitLab
• The style of these headers should match the 'Header title' style from the dropdown library spec previews.

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

I changed the dropdown global styles 😟

Why was this MR needed?

See #36409 (closed)

Screenshots (if relevant)

Project	Group	Global

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #36409 (closed)