Group required_approvals by code owners

What does this MR do and why?

This MR removes duplicate required approvals using the rule's section (e.g. Backend, Frontend) and eligible_approvers as a filter. Closes https://gitlab.com/gitlab-org/gitlab-roulette/-/issues/41.

Testing

This change is tested in gitlab-org/gitlab!92580 (comment 1028191976).

Before (gitlab-org/gitlab!85765 (comment 922034807)):

After the fix (gitlab-org/gitlab!92580 (comment 1028191976)):

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.