api-ref: deprecate images v1 api

This patch marks the Images v1 API as DEPRECATED in the api-ref.
The v1 API was deprecated in Newton by the following commit:
63e6dbb

Change-Id: Ifa71b6375a63a9ddf182448a78192d6c76ed7fba
Partial-bug: #1618495

Author: Brian Rosmaita

api-ref/source/v1/index.rst

@@ -16,9 +16,9 @@
 
 :tocdepth: 3
 
-================================
-Image Service API v1 (SUPPORTED)
-================================
+=================================
+Image Service API v1 (DEPRECATED)
+=================================
 
 .. rest_expand_all::