Deprecate citrix/netscaler module (#28280)

* Deprecate citrix/netscaler module

* 	renamed:    netscaler.py -> _netscaler.py

* Fix legacy files list

lib/ansible/modules/network/citrix/_netscaler.py

@@ -9,7 +9,7 @@ __metaclass__ = type
 
 
 ANSIBLE_METADATA = {'metadata_version': '1.1',
-                    'status': ['preview'],
+                    'status': ['deprecated'],
                     'supported_by': 'community'}
 
 
@@ -20,6 +20,7 @@ version_added: "1.1"
 short_description: Manages Citrix NetScaler entities
 description:
      - Manages Citrix NetScaler server and service entities.
+deprecated: In 2.4 use M(netscaler_service) and M(netscaler_server) instead.
 options:
   nsc_host:
     description:

test/sanity/pep8/legacy-files.txt

@@ -253,7 +253,7 @@ lib/ansible/modules/network/asa/asa_command.py
 lib/ansible/modules/network/asa/asa_config.py
 lib/ansible/modules/network/bigswitch/bigmon_chain.py
 lib/ansible/modules/network/bigswitch/bigmon_policy.py
-lib/ansible/modules/network/citrix/netscaler.py
+lib/ansible/modules/network/citrix/_netscaler.py
 lib/ansible/modules/net_tools/cloudflare_dns.py
 lib/ansible/modules/network/cumulus/_cl_bond.py
 lib/ansible/modules/network/cumulus/_cl_img_install.py