Fix form declaration

While running Drupal-check in preparation for D9 upgrade, I noticed this error reported by Drupal check

Line   src/FormScheduledUpdateSettingsForm.php
 ------ --------------------------------------------------------------------------------------------------------------
         Class Drupal\scheduled_updates\Entity\Form\ScheduledUpdateSettingsForm was not found while trying to analyse
         it - autoloading is probably not configured properly.
         💡 Learn more at https://phpstan.org/user-guide/autoloading

Doesn't seem like a D9 issue specifically, but rather a wrong form declaration or namespace or something.
It's looking for this class
Drupal\scheduled_updates\Entity\Form\ScheduledUpdateSettingsForm

But I think what it really wants is:
https://git.drupalcode.org/project/scheduled_updates/-/blob/8.x-1.x/src/...