Tone down MR Approve button when approvals are optional

Since the Approval feature was rolled out to CE, I and my colleagues have accidentally merged MRs we meant to approve on multiple occasions.

Look how close the Approve and Merge buttons are:

Proposal

This has to do with how cluttered the merge requests UI is today… 😕

But given the screenshot above, when approvals are optional, we could at least change the Approve button from a primary info variant into a default variant, like so: