Fix the issues reported by phpcs

Problem/Motivation

Getting following error/warnings.

FILE: /var/www/html/modules/contrib/scheduled_maintenance/config/schema/scheduled_maintenance.schema.yml
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FOUND 1 ERROR AFFECTING 1 LINE
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23 | ERROR | [x] Expected 1 newline at end of file; 2 found
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PHPCBF CAN FIX THE 1 MARKED SNIFF VIOLATIONS AUTOMATICALLY
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FILE: /var/www/html/modules/contrib/scheduled_maintenance/src/EventSubscriber/ScheduledMaintenanceSubscriber.php
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FOUND 1 ERROR AND 1 WARNING AFFECTING 2 LINES
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9 | WARNING | [x] Unused use statement
75 | ERROR | [ ] Type hint "\Symfony\Component\HttpKernel\Event\RequestEvent" missing for $event
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PHPCBF CAN FIX THE 1 MARKED SNIFF VIOLATIONS AUTOMATICALLY
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Time: 1.87 secs; Memory: 6MB

Steps to reproduce

Run following command

phpcs --standard=Drupal,DrupalPractice --extensions=php,module,inc,install,test,profile,theme,css,info,txt,md,yml modules/contrib/scheduled_maintenance/

Proposed resolution

Above error/warnings need to be fixed.