Add missing demodulize call to class name

What does this MR do and why?

Describe in detail what your merge request does and why.

The BackfillJiraTrackerDeploymentType2 migration job added originally via !46368 (merged) 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

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Simulate a pending job for BackfillJiraTrackerDeploymentType2 by running the following PSQL insert (change ID in first column as necessary):

   insert into background_migration_jobs values (4, '2021-02-20 06:49:53.995423+00', '2021-02-20 06:49:53.995423+00', 0, 'BackfillJiraTrackerDeploymentType2', '[1,113]');

2. Try to run the migration again, expecting it to complete and update the status table:

   Gitlab::BackgroundMigration.perform("BackfillJiraTrackerDeploymentType2", [1, 113])

3. Prior to patch, observe that the inserted record remains 0 for status column
4. After the patch, the status updates to 1 to indicate completed

More context can be found in internal slack thread on #g_database: https://gitlab.slack.com/archives/CNZ8E900G/p1644287755045969

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