Migrate to a tertiary button in DropdownContentsLabelsView

What does this MR do and why?

This migrates a link button to a tertiary one in the DropdownContentsLabelsView component.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Ensure your GDK has an GitLab Ultimate license.
2. Navigate to a group's epics list.
3. Click on Edit epics.
4. Open the Labels dropdown in the sidebar.

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.

Related to #241573 (closed)