Use mysql to set mailing.created_date

Use mysql to set mailing.created_date

On digging I determined this trigger hokiness was all about mysql
5.5 and we can ditch it now

https://dev.mysql.com/doc/relnotes/mysql/5.6/en/news-5-6-5.html#mysqld-5-6-5-data-types

and https://github.com/civicrm/civicrm-core/pull/10953#issuecomment-327613771

Note that we still support mysql 5.6 and the current version is 5.6.51

I think it's fine to require that the 5.6 minimum is 5.6.5 as released in 2012

I'll do the upgrade script as a follow up