Update button variants to btn-confirm on MR page

What does this MR do?

Makes buttons the same color to follow new GitLab UI guidelines by using btn-confirm.

Screenshots (strongly suggested)

Section	Before	After	Changes
Design management (xs)			Update button variant and alignment, fix margin to align buttons
Design management			Update button variant and alignment
MR review - Start a review (xs)			Reduce bottom margin from 10px to 8px
MR review - Start a review (md)			Fix bottom button margin when buttons break into 2 lines
MR review - Start a review			Flex align buttons to reduce margin to 8px
MR review - Review pending			Update button / badge variant

Does this MR meet the acceptance criteria?

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