GroupedListV2: update to use correct semantics when used within DetailsList

See: https://github.com/microsoft/fluentui/pull/24460#discussion_r952927882

NOTE: this is being done as a follow up so customers can get GroupedListV2 earlier to test API-compatibility with apps. Changing these semantics will not affect the API.