Allow maintainers to set MR approval settings

What does this MR do and why?

Describe in detail what your merge request does and why.

During the rollout of group-level MR approval settings, it became clear that we had accidentally removed access for project maintainers to set MR approval settings.

This MR reinstates that access.

There are no documentation changes to make because the docs are already correct and this changes the functionality to correctly match itl.

Screenshots or screen recordings (When group_merge_request_approval_settings_feature_flag is enabled)

Before

After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.