Some people may remove their birthday date after the notification was
sent, which then breaks the notification page for other users. Let's
just display the date when the notification was created, and not display
the users updated birthday date. When users update from date A to B it
always looks weird anyway, when we display the same new date B twice on
different days, or display two different dates for the same user.

We could remove notifications when users change or remove their
birthday, but that would be way more complex and also we usually don't
remove notifications (not even for deleted posts).

Fixes #7689

closes #7691