rename validation method ssn to personId

Preparation for https://github.com/cakephp/localized/issues/84

For Localized ssn() is too country specific and has been renamed to personId().
This should also be reflected in the CakePHP3.x core.

I will update the migration guide afterwards.
Should we also backport this to 2.6 with the deprecation of the ssn() method?
That would ease migration.