Removed the V2 returns announce endpoint

POST /v2/returns/announce has been removed. Announcing a return is now done
with POST /v3/returns/announce, described in
Announcing returns.

Calls to the old path answer 405 Method Not Allowed, not 404. If your
integration still posts to /v2/returns/announce, its return announcements are
not reaching byrd and no return labels are being issued for them.

Migrating requires three changes to your request:

• the path is now /v3/returns/announce
• request fields are snake_case, for example shipment_id instead of
  shipmentID
• courier is a new required field

See Announcing returns for the full field list and the
accepted carrier identifiers.