Update effective date on a group-location specialty

Updates the effective start date on a practitioner's specialty association at the group-location level, without network specificity. Use this when the effective date on an existing group-level location specialty record needs correction; for the more specific group-location-network level, use PUT /practitioners/group-practitioner-location-network-specialties/effective-date, and for the group-only level, use PUT /practitioners/group-practitioner-specialties/effective-date. The target specialty association must already exist.