Update effective date on a group-location-network specialty

Updates the effective start date on a practitioner's specialty association at the group-location-network level. Use this when the effective date on an existing group-level location-network specialty record needs correction; this operates at the most specific level of the specialty hierarchy (group, then location, then network) — for the location-only level use PUT /practitioners/group-practitioner-location-specialties/effective-date, and for the group level use PUT /practitioners/group-practitioner-specialties/effective-date. The target specialty association must already exist.