Use tertiary buttons in reviewer sidebar actions

What does this MR do and why?

Several action buttons in the merge request reviewer sidebar were styled as links (variant="link") even though they behave as buttons. This switches them to proper tertiary buttons (category="tertiary"), keeping their existing subtle appearance:

• Re-request review and Remove reviewer buttons in the reviewer list (uncollapsed_reviewer_list.vue)
• Add reviewer (+) button in the GitLab Duo Recommended reviewers section (suggested_reviewers.vue)

The inline "assign yourself" link is intentionally left as a link, since it reads as inline sentence text ("None - assign yourself").

References

None.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Open a merge request with reviewers assigned.
2. In the right sidebar, confirm the re-request and remove reviewer icons render as tertiary buttons.
3. With GitLab Duo recommended reviewers available, confirm the add (+) button renders as a tertiary button.

MR acceptance checklist

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