Add missing demodulize call to class name

The `BackfillJiraTrackerDeploymentType2` migration is remaining
stuck in pending
state at various customers because the job state update implementation
is passing the wrong string when sending a class name for lookup

The other migrations that do similar things
use the `demodulize` function
to transform a module name into just the unqualified classname but this
migration appears to have missed it

This change adds the missing method call to ensure future migration runs
(for yet-to-upgrade customers, or customers trying to re-run this) 
update the background migration jobs table correctly

lib/gitlab/background_migration/backfill_jira_tracker_deployment_type2.rb

@@ -79,7 +79,7 @@ module Gitlab
       end
 
       def mark_jobs_as_succeeded(*arguments)
-        Gitlab::Database::BackgroundMigrationJob.mark_all_as_succeeded(self.class.name, arguments)
+        Gitlab::Database::BackgroundMigrationJob.mark_all_as_succeeded(self.class.name.demodulize, arguments)
       end
     end
   end

spec/support/shared_examples/lib/gitlab/database/background_migration_job_shared_examples.rb

@@ -2,7 +2,7 @@
 
 RSpec.shared_examples 'marks background migration job records' do
   it 'marks each job record as succeeded after processing' do
-    create(:background_migration_job, class_name: "::#{described_class.name}",
+    create(:background_migration_job, class_name: "::#{described_class.name.demodulize}",
            arguments: arguments)
 
     expect(::Gitlab::Database::BackgroundMigrationJob).to receive(:mark_all_as_succeeded).and_call_original
@@ -13,7 +13,7 @@ RSpec.shared_examples 'marks background migration job records' do
   end
 
   it 'returns the number of job records marked as succeeded' do
-    create(:background_migration_job, class_name: "::#{described_class.name}",
+    create(:background_migration_job, class_name: "::#{described_class.name.demodulize}",
            arguments: arguments)
 
     jobs_updated = subject.perform(*arguments)