Fix Rake/Require offence in backup rake task

What does this MR do and why?

Contributes to: #391459 (closed)

This MR moves all requires into task definition of backup (lib/tasks/gitlab/backup.rake) rake task.

Screenshots or screen recordings

How to set up and validate locally

rake gitlab:backup:create
rake gitlab:backup:restore

bundle exec rubocop lib/tasks/gitlab/backup.rake
Inspecting 1 file
.

1 file inspected, no offenses detected

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

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