Bug 25236: Fix semantic heading for opac-full-serial-issues.tt

Bug 25236: Fix semantic heading for opac-full-serial-issues.tt

Prior to this patch, the opac-full-serial-issues side navigation box contained
semantically incorrect headings.

Test plan
1/ Search for a serial with many linked issues
2/ Navigate to the detailed view of that serial
3/ Click on the 'More details' link after all the listed issues
4/ Inspect the 'Refine your search' elements on the left side of the screen.
   Note that the headings are H1 (Koha page title) > H4 (Refine your search)
3/ Apply this patch
4/ Reload the page
5/ Re-inspect the 'Refine your search' elements and note the heading
   semantics are now H1 (Title of page) > H2 (Refine your search)
6/ Note the appearance of the box is improved and consistent with elsewhere
7/ Signoff

Signed-off-by: Sally <sally.healey@cheshiresharedservices.gov.uk>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>