Use tertiary button in MR Widget collapsible extension

What does this MR do and why?

This MR is part of the frontend pajamasintegrate initiative to migrate parts of our codebase to new pajamas components.

This MR changes button that is used for collapsable items in the MR widget (e.g. exposed artifacts) from link variant to tertiary button.

• Epic: &4232
• Storybook showing tertiary and link buttons: https://design.gitlab.com/components/button

Screenshots or screen recordings

before	after
collapsible-extension-before	collapsible-extension-after

How to set up and validate locally

1. Open an MR with Pipelines in it (in local GDK that's for example http://127.0.0.1:3000/gitlab-org/gitlab-test/-/merge_requests/1
2. You can either set up an exposed job artifact and run another pipeline.
3. Or simply apply exposed-artifact.patch patch by running git apply exposed-artifact.patch in your local gitlab project.
4. see the collapsible "View Exposed Artifact" button
   • 

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 #241568 (closed)