Bug 30061: Make Koha::Patron->get_age use DateTime math

Bug 30061: Make Koha::Patron->get_age use DateTime math

This patch does what the title says. Instead of converting dates to
strings and performing math manually, we can just use the DateTime
objects we have and calculate the DateTime::Duration there's between
them, to get the years count.

To test:
1. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Patrons.t
=> SUCCESS: Tests pass
2. Apply this patch
3. Repeat 1
=> SUCCESS: Tests pass
4. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>