Fix netconf validate capability check (#71195) (#71252)

*  Use ``:validate`` string to check if the netconf
   server supports validate capability as per netconf RFC

(cherry picked from commit 7635d23cee)

changelogs/fragments/71195-netconf_config_validate_issue.yaml

@@ -0,0 +1,2 @@
+bugfixes:
+- Fixed Ansible reporting validate not supported by netconf server when enabled in netconf - (https://github.com/ansible-collections/ansible.netcommon/issues/119).

lib/ansible/plugins/netconf/__init__.py

@@ -354,7 +354,7 @@ class NetconfBase(AnsiblePlugin):
         operations['supports_startup'] = ':startup' in capabilities
         operations['supports_xpath'] = ':xpath' in capabilities
         operations['supports_writable_running'] = ':writable-running' in capabilities
-        operations['supports_validate'] = ':writable-validate' in capabilities
+        operations['supports_validate'] = ':validate' in capabilities
 
         operations['lock_datastore'] = []
         if operations['supports_writable_running']: