Deprecate ssn() validation in favor of plugin Localized.

Follow up on https://github.com/cakephp/cakephp/pull/4116
`@deprecated 2.6 Will be removed in 3.0`.  As per phpdoc [manual](http://phpdoc.org/docs/latest/references/phpdoc/tags/deprecated.html) the format is `@deprecated [<version>] [<description>]`