Align workplan panel header buttons and actions menu icon with the new top bar redesign

What does this MR do and why?

Aligns the workplan panel header buttons and actions menu icon with the new top bar redesign:

• Changes the Edit button from category="secondary" to category="tertiary" (borderless), matching the button style used in both panels
• Changes the actions menu icon from ellipsis_v to ellipsis_h to align with the updated layout

This keeps the workplan panel visually consistent with the new top bar design.

Closes #619094 (closed)

References

• Issue: #619094 (closed)
• Epic discussion: gitlab-org#22703 (comment 3712063774)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Navigate to any work item that has a workplan (enable the workplan feature flag)
2. Open the workplan panel
3. Verify the Edit button is now borderless (tertiary style)
4. Verify the More actions dropdown uses the horizontal ellipsis icon (ellipsis_h)

MR acceptance checklist

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