Selected reviewers for an approval/CODEOWNERS should only include users of that particular rule

From #480218 (comment 2071913975)

The dropdown for a code owners rule shows all assigned reviewers, even if they are not eligible approvers for that rule. In this example, only Jeremy is an eligible approver for /contents/ but other reviewers are shown as well. I was expecting the dropdown to only list the eligible approvers for that rule.

Expected behavior

When opening the reviewer assignment dropdown of a particular rule, we should only include those users that are eligible approvers of that rule.