Use primary btn variant for finish review dropdown

What does this MR do and why?

• Related to #368799 (comment 1051209641)
• Uses primary Finish review button variant (we received feedback that users were missing the button entirely, as it used to be primary
• Shifts the Submit button to the left to align with other comments and to avoid the duplicate stacked primary buttons
• Note: I kept the text Submit review to make it more clear that the entire review is going to be submitted, not just the form content

Screenshots or screen recordings

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

• Enable feature flag :mr_review_submit_comment
• Create comment on an MR diff
• Observe primary variant of Finish review
• Click Finish review and observe left-aligned Submit review

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.