Use development-leaders as CODEOWNERS approvers

What does this MR do and why?

This MR updates the eligible CODEOWNERS file change approvers to use the sub-group gitlab-org/development-leaders instead of individual leaders' usernames.

Using this sub-group will allow us to more easily update Development department approvers by using the access request process to manage group membership.

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.