Bug 33041: Use process_tt in C4::Serial::NewIssue

Bug 33041: Use process_tt in C4::Serial::NewIssue

Bug 33030 implements a new helper subroutine to standardize processing of Template Toolkit syntax outside slips and notices. We should use this subroutine in C4::Serial::NewIssue

Test Plan:
1) Apply this patch
2) prove t/db_dependent/Koha/Items.t

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