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

Why

With the top bar redesign, the header buttons in the workplan panel no longer match the updated header layout style. Aligning them keeps the panels visually consistent.

Raised in this discussion on the epic: gitlab-org#22703 (comment 3712063774) (thread started at gitlab-org#22703 (comment 3711210237)).

What

• Update the workplan panel header buttons to align with the new top bar style:
  • Make the Edit button tertiary/borderless, matching the button style in both panels
  • Keep Implement as the primary (confirm) button
• Update the actions menu icon on the workplan from ellipsis_v to ellipsis_h to align with the updated layout