feat(GlBadge): Update guidelines for icons

Description

Previous discussion -> gitlab-org/gitlab-ui!3801 (merged)

Suggest not using md icons with sm badge and sm icons with lg badge. These two combinations look unproportional.

Suggest using the roundIcon property for badges with round icons.

Checklist

Make sure the following are completed before closing the issue:

1. Assign the correct component label to this issue.
2. Create an MR with the additions or updates needed.
3. When applicable, create an issue using the Figma update issue template to update the component in Figma, and mark it as related to this one. Bring the issue to your team planning session for prioritization and scheduling.
4. When applicable, create an MR in GitLab UI to update the component. If you do not have capacity or are unable to update the component directly, create a GitLab UI issue using the Component issue template. Mark the new issue as related to this one. Bring the issue to your team planning session for prioritization and scheduling. Mark the issue as related to this one.
5. When introducing a major or breaking change, communicate the changes within the Engineering Week in Review and UX Weekly meeting.
6. 🎉 Congrats, you made it! You can now close this issue.

gitlab-org/gitlab#430244 (closed)