Rename or describe a group value

Renames a group value account-wide and/or updates its description. At least one of new_value or description must be supplied; otherwise the API returns 400. old_value is matched case-insensitively. When a rename targets an existing value, the surviving value keeps its casing and description unless description is supplied. A missing old_value is normally a tolerant no-op; when both new_value and description are supplied, description still applies to the destination. The required action_audit object records who and what initiated the change.