Bug 40033: Call GetPlugins correctly in background_jobs.pl

Test plan:

1. Install a plugin that does not have any background_tasks, e.g. https://github.com/HKS3/HKS3GeoSearch/releases/tag/v0.12
2. Navigate to http://localhost:8081/cgi-bin/koha/admin/background_jobs.pl, observe a 500
3. Apply patch
4. Observe no 500

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: David Cook <dcook@prosentient.com.au>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
(cherry picked from commit 53630add0d)
Signed-off-by: Paul Derscheid <paul.derscheid@lmscloud.de>

admin/background_jobs.pl

@@ -79,7 +79,7 @@ $template->param(
     op       => $op,
 );
 
-my @plugins = Koha::Plugins::GetPlugins(
+my @plugins = Koha::Plugins->new()->GetPlugins(
     {
         method => 'background_tasks',
     }