Fix Group Import members' owner access level

What does this MR do?

This MR fixes a bug for Group Import where imported group members who had Owner access level were lowered down to Maintainer. This is due to the fact that MembersMapper was initially for Projects only (which has max access level of Maintainer) and it was later on changed to accommodate Groups. But max access level was not touched - hence all Owners of groups are lowered to Maintainers.

This change fixes that, allowing Group members to retain Owner access level.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

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