Update the api-ref to mark the v2 API as deprecated

The v2 API was deprecated in mitaka with change:

e63a8311fa

The api-ref needs to be updated to show that as
well since that's the main thing someone sees when
they are in the API docs.

The status value comes from:

https://wiki.openstack.org/wiki/VersionDiscovery#status

Change-Id: I6dc5d8fd26d0981e5d83e829fe2dd09b7b9b4c17

api-ref/source/v2-admin/index.rst

@@ -1,7 +1,7 @@
 :tocdepth: 2
 
 =====================================
- Identity Admin API v2.0 (SUPPORTED)
+ Identity Admin API v2.0 (DEPRECATED)
 =====================================
 
 .. rest_expand_all::

api-ref/source/v2-ext/index.rst

@@ -1,7 +1,7 @@
 :tocdepth: 2
 
 ==========================================
- Identity API v2.0 extensions (SUPPORTED)
+ Identity API v2.0 extensions (DEPRECATED)
 ==========================================
 
 .. rest_expand_all::

api-ref/source/v2/index.rst

@@ -1,7 +1,7 @@
 :tocdepth: 2
 
 ===============================
- Identity API v2.0 (SUPPORTED)
+ Identity API v2.0 (DEPRECATED)
 ===============================
 
 .. rest_expand_all::